diff --git a/lib/multi-tk.rb b/lib/multi-tk.rb
index d291c926..e1f261f2 100644
--- a/lib/multi-tk.rb
+++ b/lib/multi-tk.rb
@@ -3519,4 +3519,4 @@ class MultiTkIp
 module TkCore
   INTERP = MultiTkIp
 end
-require 'tk'
+require 'tk' unless defined?(Tk)
diff --git a/lib/thread_tk.rb b/lib/thread_tk.rb
index 69ed9386..54b75ae6 100644
--- a/lib/thread_tk.rb
+++ b/lib/thread_tk.rb
@@ -10,4 +10,4 @@
 end
 
 module TkCore; RUN_EVENTLOOP_ON_MAIN_THREAD = false; end
-require 'tk'
+require 'tk' unless defined?(Tk)
diff --git a/lib/tk.rb b/lib/tk.rb
index 77704931..1d84d93f 100644
--- a/lib/tk.rb
+++ b/lib/tk.rb
@@ -1317,8 +1317,10 @@ def @tk_cmd_tbl.[]=(idx,val)
       @add_tk_procs = []  # table of [name, args, body]
 
       @force_default_encoding ||= [false]
-      @encoding ||= [nil]
-      def @encoding.to_s; self.join(nil); end
+      unless @encoding
+        @encoding = [nil]
+        def @encoding.to_s; self.join(nil); end
+      end
 
       @cb_entry_class = Class.new(TkCallbackEntry){
         class << self
diff --git a/lib/tk/bgerror.rb b/lib/tk/bgerror.rb
index 7b4f625e..f5629ba8 100644
--- a/lib/tk/bgerror.rb
+++ b/lib/tk/bgerror.rb
@@ -3,7 +3,7 @@
 #  tkbgerror -- bgerror ( tkerror ) module
 #                     1998/07/16 by Hidetoshi Nagai <nagai@ai.kyutech.ac.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkBgError
   extend Tk
diff --git a/lib/tk/bindtag.rb b/lib/tk/bindtag.rb
index 71b80395..3789eaa0 100644
--- a/lib/tk/bindtag.rb
+++ b/lib/tk/bindtag.rb
@@ -2,7 +2,7 @@
 #
 # tk/bind.rb : control event binding
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkBindTag
   include TkBindCore
diff --git a/lib/tk/busy.rb b/lib/tk/busy.rb
index 18ebede9..a5c7c863 100644
--- a/lib/tk/busy.rb
+++ b/lib/tk/busy.rb
@@ -2,7 +2,7 @@
 #
 # tk/busy.rb: support 'tk busy' command (Tcl/Tk8.6 or later)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk::Busy
   include TkCore
diff --git a/lib/tk/button.rb b/lib/tk/button.rb
index dc252d8e..7a800d52 100644
--- a/lib/tk/button.rb
+++ b/lib/tk/button.rb
@@ -2,7 +2,7 @@
 #
 # tk/button.rb : treat button widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/label'
 
 class Tk::Button<Tk::Label
diff --git a/lib/tk/canvas.rb b/lib/tk/canvas.rb
index b83bc9c1..cd6c70cf 100644
--- a/lib/tk/canvas.rb
+++ b/lib/tk/canvas.rb
@@ -3,7 +3,7 @@
 #               tk/canvas.rb - Tk canvas classes
 #                       by Yukihiro Matsumoto <matz@caelum.co.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/canvastag'
 require 'tk/itemconfig'
 require 'tk/scrollable'
diff --git a/lib/tk/canvastag.rb b/lib/tk/canvastag.rb
index 9aede092..6f777de1 100644
--- a/lib/tk/canvastag.rb
+++ b/lib/tk/canvastag.rb
@@ -2,7 +2,7 @@
 #
 # tk/canvastag.rb - methods for treating canvas tags
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/tagfont'
 
 module TkcTagAccess
diff --git a/lib/tk/checkbutton.rb b/lib/tk/checkbutton.rb
index 521b015b..5848714e 100644
--- a/lib/tk/checkbutton.rb
+++ b/lib/tk/checkbutton.rb
@@ -2,7 +2,7 @@
 #
 # tk/checkbutton.rb : treat checkbutton widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/radiobutton'
 
 class Tk::CheckButton<Tk::RadioButton
diff --git a/lib/tk/clipboard.rb b/lib/tk/clipboard.rb
index c5f481f9..6ebb0ad5 100644
--- a/lib/tk/clipboard.rb
+++ b/lib/tk/clipboard.rb
@@ -2,7 +2,7 @@
 #
 # tk/clipboard.rb : methods to treat clipboard
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkClipboard
   include Tk
diff --git a/lib/tk/clock.rb b/lib/tk/clock.rb
index df900d73..2e533aa2 100644
--- a/lib/tk/clock.rb
+++ b/lib/tk/clock.rb
@@ -2,7 +2,7 @@
 #
 # tk/clock.rb : methods for clock command
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk
   module Clock
diff --git a/lib/tk/composite.rb b/lib/tk/composite.rb
index 830d383d..4c34839e 100644
--- a/lib/tk/composite.rb
+++ b/lib/tk/composite.rb
@@ -2,7 +2,7 @@
 #
 # tk/composite.rb :
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkComposite
   include Tk
diff --git a/lib/tk/console.rb b/lib/tk/console.rb
index c560912d..79b9a735 100644
--- a/lib/tk/console.rb
+++ b/lib/tk/console.rb
@@ -2,7 +2,7 @@
 #
 #   tk/console.rb : control the console on system without a real console
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkConsole
   include Tk
diff --git a/lib/tk/dialog.rb b/lib/tk/dialog.rb
index 0b19be7c..38ad88c9 100644
--- a/lib/tk/dialog.rb
+++ b/lib/tk/dialog.rb
@@ -2,7 +2,7 @@
 #
 #   tk/dialog.rb : create dialog boxes
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/variable.rb'
 
 class TkDialogObj < TkWindow
diff --git a/lib/tk/encodedstr.rb b/lib/tk/encodedstr.rb
index 9ca13b3d..f55a56ae 100644
--- a/lib/tk/encodedstr.rb
+++ b/lib/tk/encodedstr.rb
@@ -2,7 +2,7 @@
 #
 # tk/encodedstr.rb : Tk::EncodedString class
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 ###########################################
 #  string with Tcl's encoding
diff --git a/lib/tk/entry.rb b/lib/tk/entry.rb
index a2ae093e..3fa2df80 100644
--- a/lib/tk/entry.rb
+++ b/lib/tk/entry.rb
@@ -3,7 +3,7 @@
 #               tk/entry.rb - Tk entry classes
 #                       by Yukihiro Matsumoto <matz@caelum.co.jp>
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/label'
 require 'tk/scrollable'
 require 'tk/validation'
diff --git a/lib/tk/font.rb b/lib/tk/font.rb
index 3c4475d1..c76481cb 100644
--- a/lib/tk/font.rb
+++ b/lib/tk/font.rb
@@ -4,7 +4,7 @@
 #
 #                               by  Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkFont
   include Tk
diff --git a/lib/tk/fontchooser.rb b/lib/tk/fontchooser.rb
index fb220da2..01212260 100644
--- a/lib/tk/fontchooser.rb
+++ b/lib/tk/fontchooser.rb
@@ -2,7 +2,7 @@
 #
 # tk/fontchooser.rb -- "tk fontchooser" support (Tcl/Tk8.6 or later)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/font'
 
 module TkFont::Chooser
diff --git a/lib/tk/frame.rb b/lib/tk/frame.rb
index 0682faf3..dbac6e7f 100644
--- a/lib/tk/frame.rb
+++ b/lib/tk/frame.rb
@@ -2,7 +2,7 @@
 #
 # tk/frame.rb : treat frame widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class Tk::Frame<TkWindow
   TkCommandNames = ['frame'.freeze].freeze
diff --git a/lib/tk/grid.rb b/lib/tk/grid.rb
index 0c35b957..27aa0766 100644
--- a/lib/tk/grid.rb
+++ b/lib/tk/grid.rb
@@ -2,7 +2,7 @@
 #
 # tk/grid.rb : control grid geometry manager
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkGrid
   include Tk
diff --git a/lib/tk/image.rb b/lib/tk/image.rb
index 673f247c..485d837f 100644
--- a/lib/tk/image.rb
+++ b/lib/tk/image.rb
@@ -3,7 +3,7 @@
 # tk/image.rb : treat Tk image objects
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkImage<TkObject
   include Tk
diff --git a/lib/tk/itemconfig.rb b/lib/tk/itemconfig.rb
index 89d6858d..1275822e 100644
--- a/lib/tk/itemconfig.rb
+++ b/lib/tk/itemconfig.rb
@@ -2,7 +2,7 @@
 #
 # tk/itemconfig.rb : control item/tag configuration of widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkutil'
 require 'tk/itemfont.rb'
 
diff --git a/lib/tk/itemfont.rb b/lib/tk/itemfont.rb
index 32e0ea95..329baffe 100644
--- a/lib/tk/itemfont.rb
+++ b/lib/tk/itemfont.rb
@@ -2,7 +2,7 @@
 #
 # tk/itemfont.rb : control font of widget items
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkItemFontOptkeys
   def __item_font_optkeys(id)
diff --git a/lib/tk/kinput.rb b/lib/tk/kinput.rb
index 6d2b3d7a..9e07868b 100644
--- a/lib/tk/kinput.rb
+++ b/lib/tk/kinput.rb
@@ -2,7 +2,7 @@
 #
 # tk/kinput.rb : control kinput
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkKinput
   include Tk
diff --git a/lib/tk/label.rb b/lib/tk/label.rb
index 48f67deb..00215657 100644
--- a/lib/tk/label.rb
+++ b/lib/tk/label.rb
@@ -2,7 +2,7 @@
 #
 # tk/label.rb : treat label widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class Tk::Label<TkWindow
   TkCommandNames = ['label'.freeze].freeze
diff --git a/lib/tk/labelframe.rb b/lib/tk/labelframe.rb
index f9497dc2..7ab79a6d 100644
--- a/lib/tk/labelframe.rb
+++ b/lib/tk/labelframe.rb
@@ -2,7 +2,7 @@
 #
 # tk/labelframe.rb : treat labelframe widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 
 class Tk::LabelFrame<Tk::Frame
diff --git a/lib/tk/listbox.rb b/lib/tk/listbox.rb
index f8cc0e99..34a94c1f 100644
--- a/lib/tk/listbox.rb
+++ b/lib/tk/listbox.rb
@@ -2,7 +2,7 @@
 #
 # tk/listbox.rb : treat listbox widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/itemconfig'
 require 'tk/scrollable'
 require 'tk/txtwin_abst'
diff --git a/lib/tk/macpkg.rb b/lib/tk/macpkg.rb
index abf9b176..b5e8e6ad 100644
--- a/lib/tk/macpkg.rb
+++ b/lib/tk/macpkg.rb
@@ -7,7 +7,7 @@
 #         This is NOT TESTED. Because I have no test-environment.
 #
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk
   def Tk.load_tclscript_rsrc(resource_name, file=None)
diff --git a/lib/tk/menu.rb b/lib/tk/menu.rb
index c78c21a5..55ff88e9 100644
--- a/lib/tk/menu.rb
+++ b/lib/tk/menu.rb
@@ -2,7 +2,7 @@
 #
 # tk/menu.rb : treat menu and menubutton
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/itemconfig'
 require 'tk/menuspec'
 
diff --git a/lib/tk/menubar.rb b/lib/tk/menubar.rb
index 73f8801d..5097b239 100644
--- a/lib/tk/menubar.rb
+++ b/lib/tk/menubar.rb
@@ -85,7 +85,7 @@
 # To use add_menu, configuration must be done by calling configure after
 # adding all menus by add_menu, not by the constructor arguments.
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tk/composite'
 require 'tk/menuspec'
diff --git a/lib/tk/message.rb b/lib/tk/message.rb
index 432e9667..c2e950c2 100644
--- a/lib/tk/message.rb
+++ b/lib/tk/message.rb
@@ -2,7 +2,7 @@
 #
 # tk/message.rb : treat message widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/label'
 
 class Tk::Message<Tk::Label
diff --git a/lib/tk/mngfocus.rb b/lib/tk/mngfocus.rb
index 06b20eb6..c406e2b1 100644
--- a/lib/tk/mngfocus.rb
+++ b/lib/tk/mngfocus.rb
@@ -3,7 +3,7 @@
 #   tk/mngfocus.rb : methods for Tcl/Tk standard library 'focus.tcl'
 #                           by Hidetoshi Nagai <nagai@ai.kyutech.ac.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkManageFocus
   extend Tk
diff --git a/lib/tk/msgcat.rb b/lib/tk/msgcat.rb
index 486941d2..29c1dcea 100644
--- a/lib/tk/msgcat.rb
+++ b/lib/tk/msgcat.rb
@@ -3,7 +3,7 @@
 #   tk/msgcat.rb : methods for Tcl message catalog
 #                     by Hidetoshi Nagai <nagai@ai.kyutech.ac.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 #class TkMsgCatalog
 class TkMsgCatalog < TkObject
diff --git a/lib/tk/namespace.rb b/lib/tk/namespace.rb
index aa09fb36..313a0799 100644
--- a/lib/tk/namespace.rb
+++ b/lib/tk/namespace.rb
@@ -3,7 +3,7 @@
 #   tk/namespace.rb : methods to manipulate Tcl/Tk namespace
 #                           by Hidetoshi Nagai <nagai@ai.kyutech.ac.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkNamespace < TkObject
   extend Tk
diff --git a/lib/tk/optiondb.rb b/lib/tk/optiondb.rb
index 34d49aae..d8e66c5a 100644
--- a/lib/tk/optiondb.rb
+++ b/lib/tk/optiondb.rb
@@ -2,7 +2,7 @@
 #
 # tk/optiondb.rb : treat option database
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkOptionDB
   include Tk
diff --git a/lib/tk/optionobj.rb b/lib/tk/optionobj.rb
index 28f957fa..a258248b 100644
--- a/lib/tk/optionobj.rb
+++ b/lib/tk/optionobj.rb
@@ -8,7 +8,7 @@
 #        e.g. hash_kv({'aaa'=>1, 'bbb'=>Tk::None, 'ccc'=>3})
 #                 => ["-aaa", 1, "-bbb", "-ccc", 3]
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk
   class OptionObj < Hash
diff --git a/lib/tk/pack.rb b/lib/tk/pack.rb
index 45926d40..9aad2b05 100644
--- a/lib/tk/pack.rb
+++ b/lib/tk/pack.rb
@@ -2,7 +2,7 @@
 #
 # tk/pack.rb : control pack geometry manager
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkPack
   include Tk
diff --git a/lib/tk/package.rb b/lib/tk/package.rb
index fe6939c2..4ded5c96 100644
--- a/lib/tk/package.rb
+++ b/lib/tk/package.rb
@@ -2,7 +2,7 @@
 #
 # tk/package.rb : package command
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkPackage
   include TkCore
diff --git a/lib/tk/palette.rb b/lib/tk/palette.rb
index e118e8db..a8b560f7 100644
--- a/lib/tk/palette.rb
+++ b/lib/tk/palette.rb
@@ -3,7 +3,7 @@
 #   tk/palette.rb : methods for Tcl/Tk standard library 'palette.tcl'
 #                     1998/06/21 by Hidetoshi Nagai <nagai@ai.kyutech.ac.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkPalette
   include Tk
diff --git a/lib/tk/panedwindow.rb b/lib/tk/panedwindow.rb
index 285292f0..a8693856 100644
--- a/lib/tk/panedwindow.rb
+++ b/lib/tk/panedwindow.rb
@@ -2,7 +2,7 @@
 #
 # tk/panedwindow.rb : treat panedwindow
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class Tk::PanedWindow<TkWindow
   TkCommandNames = ['panedwindow'.freeze].freeze
diff --git a/lib/tk/place.rb b/lib/tk/place.rb
index efda36df..68cea7e6 100644
--- a/lib/tk/place.rb
+++ b/lib/tk/place.rb
@@ -2,7 +2,7 @@
 #
 # tk/place.rb : control place geometry manager
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkPlace
   include Tk
diff --git a/lib/tk/radiobutton.rb b/lib/tk/radiobutton.rb
index b0858df2..bb30cd1f 100644
--- a/lib/tk/radiobutton.rb
+++ b/lib/tk/radiobutton.rb
@@ -2,7 +2,7 @@
 #
 # tk/radiobutton.rb : treat radiobutton widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/button'
 
 class Tk::RadioButton<Tk::Button
diff --git a/lib/tk/root.rb b/lib/tk/root.rb
index b92732fd..2a1fb269 100644
--- a/lib/tk/root.rb
+++ b/lib/tk/root.rb
@@ -2,7 +2,7 @@
 #
 # tk/root.rb : treat root widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/wm'
 require 'tk/menuspec'
 
diff --git a/lib/tk/scale.rb b/lib/tk/scale.rb
index 5ff3ff08..251c10ef 100644
--- a/lib/tk/scale.rb
+++ b/lib/tk/scale.rb
@@ -2,7 +2,7 @@
 #
 # tk/scale.rb : treat scale widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class Tk::Scale<TkWindow
   TkCommandNames = ['scale'.freeze].freeze
diff --git a/lib/tk/scrollable.rb b/lib/tk/scrollable.rb
index 9f65ece1..4c2405a1 100644
--- a/lib/tk/scrollable.rb
+++ b/lib/tk/scrollable.rb
@@ -2,7 +2,7 @@
 #
 # tk/scrollable.rb : module for scrollable widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk
   module XScrollable
diff --git a/lib/tk/scrollbar.rb b/lib/tk/scrollbar.rb
index c16ed387..1f14e3c5 100644
--- a/lib/tk/scrollbar.rb
+++ b/lib/tk/scrollbar.rb
@@ -2,7 +2,7 @@
 #
 # tk/scrollbar.rb : treat scrollbar widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class Tk::Scrollbar<TkWindow
   TkCommandNames = ['scrollbar'.freeze].freeze
diff --git a/lib/tk/scrollbox.rb b/lib/tk/scrollbox.rb
index a470765d..c5c95951 100644
--- a/lib/tk/scrollbox.rb
+++ b/lib/tk/scrollbox.rb
@@ -4,7 +4,7 @@
 #                                 as an example of Composite Widget
 #                       by Yukihiro Matsumoto <matz@netlab.co.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/listbox'
 
 class TkScrollbox<Tk::Listbox
diff --git a/lib/tk/selection.rb b/lib/tk/selection.rb
index 11196b90..65746bdd 100644
--- a/lib/tk/selection.rb
+++ b/lib/tk/selection.rb
@@ -2,7 +2,7 @@
 #
 # tk/selection.rb : control selection
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkSelection
   include Tk
diff --git a/lib/tk/spinbox.rb b/lib/tk/spinbox.rb
index 4be41c42..bf1228aa 100644
--- a/lib/tk/spinbox.rb
+++ b/lib/tk/spinbox.rb
@@ -3,7 +3,7 @@
 #               tk/spinbox.rb - Tk spinbox classes
 #                       by Yukihiro Matsumoto <matz@caelum.co.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/entry'
 
 class Tk::Spinbox<Tk::Entry
diff --git a/lib/tk/tagfont.rb b/lib/tk/tagfont.rb
index e3fd6046..785d8f82 100644
--- a/lib/tk/tagfont.rb
+++ b/lib/tk/tagfont.rb
@@ -2,7 +2,7 @@
 #
 # tk/tagfont.rb : control font of tags
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkTreatTagFont
   def font_configinfo
diff --git a/lib/tk/text.rb b/lib/tk/text.rb
index 9d9267c4..ebb7705e 100644
--- a/lib/tk/text.rb
+++ b/lib/tk/text.rb
@@ -2,7 +2,7 @@
 #
 #               tk/text.rb - Tk text classes
 #                       by Yukihiro Matsumoto <matz@caelum.co.jp>
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/itemfont'
 require 'tk/itemconfig'
 require 'tk/scrollable'
diff --git a/lib/tk/textimage.rb b/lib/tk/textimage.rb
index f7329c5d..a4b959e0 100644
--- a/lib/tk/textimage.rb
+++ b/lib/tk/textimage.rb
@@ -2,7 +2,7 @@
 #
 # tk/textimage.rb - treat Tk text image object
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/text'
 
 class TkTextImage<TkObject
diff --git a/lib/tk/textmark.rb b/lib/tk/textmark.rb
index e36dc7cf..c5179dc2 100644
--- a/lib/tk/textmark.rb
+++ b/lib/tk/textmark.rb
@@ -2,7 +2,7 @@
 #
 # tk/textmark.rb - methods for treating text marks
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/text'
 
 class TkTextMark<TkObject
diff --git a/lib/tk/texttag.rb b/lib/tk/texttag.rb
index 2b376616..e4fc3840 100644
--- a/lib/tk/texttag.rb
+++ b/lib/tk/texttag.rb
@@ -2,7 +2,7 @@
 #
 # tk/texttag.rb - methods for treating text tags
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/text'
 require 'tk/tagfont'
 
diff --git a/lib/tk/textwindow.rb b/lib/tk/textwindow.rb
index e0be5bac..a201de5a 100644
--- a/lib/tk/textwindow.rb
+++ b/lib/tk/textwindow.rb
@@ -2,7 +2,7 @@
 #
 # tk/textwindow.rb - treat Tk text window object
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/text'
 
 class TkTextWindow<TkObject
diff --git a/lib/tk/timer.rb b/lib/tk/timer.rb
index 57603292..a6094d72 100644
--- a/lib/tk/timer.rb
+++ b/lib/tk/timer.rb
@@ -4,7 +4,7 @@
 #
 #   $Id$
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkTimer
   include TkCore
diff --git a/lib/tk/tk_mac.rb b/lib/tk/tk_mac.rb
index f306c4fc..4ce2536e 100644
--- a/lib/tk/tk_mac.rb
+++ b/lib/tk/tk_mac.rb
@@ -6,7 +6,7 @@
 #     ATTENTION !!
 #         This is NOT TESTED. Because I have no test-environment.
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk
   module Mac
diff --git a/lib/tk/toplevel.rb b/lib/tk/toplevel.rb
index e66b0fc1..86d87b92 100644
--- a/lib/tk/toplevel.rb
+++ b/lib/tk/toplevel.rb
@@ -2,7 +2,7 @@
 #
 # tk/toplevel.rb : treat toplevel widget
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/wm'
 require 'tk/menuspec'
 
diff --git a/lib/tk/txtwin_abst.rb b/lib/tk/txtwin_abst.rb
index 32986d47..de49e4b6 100644
--- a/lib/tk/txtwin_abst.rb
+++ b/lib/tk/txtwin_abst.rb
@@ -2,7 +2,7 @@
 #
 # tk/txtwin_abst.rb : TkTextWin abstract class
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkTextWin<TkWindow
   TkCommandNames = [].freeze
diff --git a/lib/tk/validation.rb b/lib/tk/validation.rb
index ea30df47..e049b561 100644
--- a/lib/tk/validation.rb
+++ b/lib/tk/validation.rb
@@ -2,7 +2,7 @@
 #
 #  tk/validation.rb - validation support module for entry, spinbox, and so on
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk
   module ValidateConfigure
diff --git a/lib/tk/variable.rb b/lib/tk/variable.rb
index 4afa89c7..4897c45b 100644
--- a/lib/tk/variable.rb
+++ b/lib/tk/variable.rb
@@ -2,7 +2,7 @@
 #
 # tk/variable.rb : treat Tk variable object
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkVariable
   include Tk
diff --git a/lib/tk/virtevent.rb b/lib/tk/virtevent.rb
index 31f52c3f..c31c7655 100644
--- a/lib/tk/virtevent.rb
+++ b/lib/tk/virtevent.rb
@@ -3,7 +3,7 @@
 #   tk/virtevent.rb : treats virtual events
 #                     1998/07/16 by Hidetoshi Nagai <nagai@ai.kyutech.ac.jp>
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 class TkVirtualEvent<TkObject
   extend Tk
diff --git a/lib/tk/winfo.rb b/lib/tk/winfo.rb
index 5cfa8363..25c501a6 100644
--- a/lib/tk/winfo.rb
+++ b/lib/tk/winfo.rb
@@ -5,7 +5,7 @@
 module TkWinfo
 end
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkWinfo
   include Tk
diff --git a/lib/tk/winpkg.rb b/lib/tk/winpkg.rb
index 2a8f724b..c9d56235 100644
--- a/lib/tk/winpkg.rb
+++ b/lib/tk/winpkg.rb
@@ -6,7 +6,7 @@
 #     ATTENTION !!
 #         This is NOT TESTED. Because I have no test-environment.
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk::WinDDE
 end
diff --git a/lib/tk/wm.rb b/lib/tk/wm.rb
index 5aa67e37..824af2a1 100644
--- a/lib/tk/wm.rb
+++ b/lib/tk/wm.rb
@@ -2,7 +2,7 @@
 #
 # tk/wm.rb : methods for wm command
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module Tk
   module Wm
diff --git a/lib/tk/xim.rb b/lib/tk/xim.rb
index 08c7d0c0..240e7da7 100644
--- a/lib/tk/xim.rb
+++ b/lib/tk/xim.rb
@@ -2,7 +2,7 @@
 #
 # tk/xim.rb : control input_method
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 module TkXIM
   include Tk
diff --git a/lib/tkextlib/ICONS/icons.rb b/lib/tkextlib/ICONS/icons.rb
index 2934757b..6c62788c 100644
--- a/lib/tkextlib/ICONS/icons.rb
+++ b/lib/tkextlib/ICONS/icons.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/blt.rb b/lib/tkextlib/blt.rb
index 665e4122..ef9fc75f 100644
--- a/lib/tkextlib/blt.rb
+++ b/lib/tkextlib/blt.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/variable'
 
 # call setup script for general 'tkextlib' libraries
diff --git a/lib/tkextlib/blt/barchart.rb b/lib/tkextlib/blt/barchart.rb
index fd0ec383..c0d596ba 100644
--- a/lib/tkextlib/blt/barchart.rb
+++ b/lib/tkextlib/blt/barchart.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 require 'tkextlib/blt/component.rb'
 
diff --git a/lib/tkextlib/blt/bitmap.rb b/lib/tkextlib/blt/bitmap.rb
index fa783765..b405ad61 100644
--- a/lib/tkextlib/blt/bitmap.rb
+++ b/lib/tkextlib/blt/bitmap.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/busy.rb b/lib/tkextlib/blt/busy.rb
index ab244207..c11e290c 100644
--- a/lib/tkextlib/blt/busy.rb
+++ b/lib/tkextlib/blt/busy.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/itemconfig.rb'
 require 'tkextlib/blt.rb'
 
diff --git a/lib/tkextlib/blt/component.rb b/lib/tkextlib/blt/component.rb
index 1d7b217b..f2b96506 100644
--- a/lib/tkextlib/blt/component.rb
+++ b/lib/tkextlib/blt/component.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/container.rb b/lib/tkextlib/blt/container.rb
index 6ee11138..c92d350e 100644
--- a/lib/tkextlib/blt/container.rb
+++ b/lib/tkextlib/blt/container.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/cutbuffer.rb b/lib/tkextlib/blt/cutbuffer.rb
index b36aece6..bd471e76 100644
--- a/lib/tkextlib/blt/cutbuffer.rb
+++ b/lib/tkextlib/blt/cutbuffer.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/dragdrop.rb b/lib/tkextlib/blt/dragdrop.rb
index 1b116de8..ca8bedc1 100644
--- a/lib/tkextlib/blt/dragdrop.rb
+++ b/lib/tkextlib/blt/dragdrop.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/itemconfig'
 require 'tkextlib/blt.rb'
 
diff --git a/lib/tkextlib/blt/eps.rb b/lib/tkextlib/blt/eps.rb
index bd2d3cc4..afd9e8cc 100644
--- a/lib/tkextlib/blt/eps.rb
+++ b/lib/tkextlib/blt/eps.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/canvas'
 require 'tkextlib/blt.rb'
 
diff --git a/lib/tkextlib/blt/graph.rb b/lib/tkextlib/blt/graph.rb
index e6c5c6d9..f3f7f97e 100644
--- a/lib/tkextlib/blt/graph.rb
+++ b/lib/tkextlib/blt/graph.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 require 'tkextlib/blt/component.rb'
 
diff --git a/lib/tkextlib/blt/htext.rb b/lib/tkextlib/blt/htext.rb
index d0f5e8b5..fb9a7d00 100644
--- a/lib/tkextlib/blt/htext.rb
+++ b/lib/tkextlib/blt/htext.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/itemconfig.rb'
 require 'tkextlib/blt.rb'
 
diff --git a/lib/tkextlib/blt/spline.rb b/lib/tkextlib/blt/spline.rb
index 84d96a08..61cd560d 100644
--- a/lib/tkextlib/blt/spline.rb
+++ b/lib/tkextlib/blt/spline.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/stripchart.rb b/lib/tkextlib/blt/stripchart.rb
index f485c072..2b2700ba 100644
--- a/lib/tkextlib/blt/stripchart.rb
+++ b/lib/tkextlib/blt/stripchart.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 require 'tkextlib/blt/component.rb'
 
diff --git a/lib/tkextlib/blt/table.rb b/lib/tkextlib/blt/table.rb
index 39e34540..7cbc1d24 100644
--- a/lib/tkextlib/blt/table.rb
+++ b/lib/tkextlib/blt/table.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/itemconfig.rb'
 require 'tkextlib/blt.rb'
 
diff --git a/lib/tkextlib/blt/tabnotebook.rb b/lib/tkextlib/blt/tabnotebook.rb
index f08f1981..4219c99c 100644
--- a/lib/tkextlib/blt/tabnotebook.rb
+++ b/lib/tkextlib/blt/tabnotebook.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 require 'tkextlib/blt/tabset.rb'
 
diff --git a/lib/tkextlib/blt/tabset.rb b/lib/tkextlib/blt/tabset.rb
index 69afa200..a2349e9c 100644
--- a/lib/tkextlib/blt/tabset.rb
+++ b/lib/tkextlib/blt/tabset.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/ted.rb b/lib/tkextlib/blt/ted.rb
index 7e81eab2..ab9b546b 100644
--- a/lib/tkextlib/blt/ted.rb
+++ b/lib/tkextlib/blt/ted.rb
@@ -7,7 +7,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/tile.rb b/lib/tkextlib/blt/tile.rb
index f52a5d39..a653c2b2 100644
--- a/lib/tkextlib/blt/tile.rb
+++ b/lib/tkextlib/blt/tile.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/tile/button.rb b/lib/tkextlib/blt/tile/button.rb
index e3d819ed..ff6a5264 100644
--- a/lib/tkextlib/blt/tile/button.rb
+++ b/lib/tkextlib/blt/tile/button.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/button'
 require 'tkextlib/blt/tile.rb'
 
diff --git a/lib/tkextlib/blt/tile/checkbutton.rb b/lib/tkextlib/blt/tile/checkbutton.rb
index 0c8c7f97..78e6b28f 100644
--- a/lib/tkextlib/blt/tile/checkbutton.rb
+++ b/lib/tkextlib/blt/tile/checkbutton.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/checkbutton'
 require 'tkextlib/blt/tile.rb'
 
diff --git a/lib/tkextlib/blt/tile/frame.rb b/lib/tkextlib/blt/tile/frame.rb
index 8d979fe4..0b82e43b 100644
--- a/lib/tkextlib/blt/tile/frame.rb
+++ b/lib/tkextlib/blt/tile/frame.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/blt/tile.rb'
 
diff --git a/lib/tkextlib/blt/tile/label.rb b/lib/tkextlib/blt/tile/label.rb
index f58450d9..9bc9ba53 100644
--- a/lib/tkextlib/blt/tile/label.rb
+++ b/lib/tkextlib/blt/tile/label.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/label'
 require 'tkextlib/blt/tile.rb'
 
diff --git a/lib/tkextlib/blt/tile/radiobutton.rb b/lib/tkextlib/blt/tile/radiobutton.rb
index ee449c75..6205bc09 100644
--- a/lib/tkextlib/blt/tile/radiobutton.rb
+++ b/lib/tkextlib/blt/tile/radiobutton.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/radiobutton'
 require 'tkextlib/blt/tile.rb'
 
diff --git a/lib/tkextlib/blt/tile/scrollbar.rb b/lib/tkextlib/blt/tile/scrollbar.rb
index 35d878d5..da3449af 100644
--- a/lib/tkextlib/blt/tile/scrollbar.rb
+++ b/lib/tkextlib/blt/tile/scrollbar.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/scrollbar'
 require 'tkextlib/blt/tile.rb'
 
diff --git a/lib/tkextlib/blt/tile/toplevel.rb b/lib/tkextlib/blt/tile/toplevel.rb
index e30b8b37..fb4d9aa6 100644
--- a/lib/tkextlib/blt/tile/toplevel.rb
+++ b/lib/tkextlib/blt/tile/toplevel.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/toplevel'
 require 'tkextlib/blt/tile.rb'
 
diff --git a/lib/tkextlib/blt/tree.rb b/lib/tkextlib/blt/tree.rb
index 7539b527..2ea961fd 100644
--- a/lib/tkextlib/blt/tree.rb
+++ b/lib/tkextlib/blt/tree.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/treeview.rb b/lib/tkextlib/blt/treeview.rb
index 3b93e406..12af720f 100644
--- a/lib/tkextlib/blt/treeview.rb
+++ b/lib/tkextlib/blt/treeview.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 require 'tk/validation.rb'
 
diff --git a/lib/tkextlib/blt/unix_dnd.rb b/lib/tkextlib/blt/unix_dnd.rb
index 3c0cd33b..d606b30d 100644
--- a/lib/tkextlib/blt/unix_dnd.rb
+++ b/lib/tkextlib/blt/unix_dnd.rb
@@ -7,7 +7,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/vector.rb b/lib/tkextlib/blt/vector.rb
index a3ab7e64..3ea67716 100644
--- a/lib/tkextlib/blt/vector.rb
+++ b/lib/tkextlib/blt/vector.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/watch.rb b/lib/tkextlib/blt/watch.rb
index 9ec6f269..c5fd103d 100644
--- a/lib/tkextlib/blt/watch.rb
+++ b/lib/tkextlib/blt/watch.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/win_printer.rb b/lib/tkextlib/blt/win_printer.rb
index 8d609acc..7952be6a 100644
--- a/lib/tkextlib/blt/win_printer.rb
+++ b/lib/tkextlib/blt/win_printer.rb
@@ -7,7 +7,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/blt/winop.rb b/lib/tkextlib/blt/winop.rb
index 38033e11..6c3468b1 100644
--- a/lib/tkextlib/blt/winop.rb
+++ b/lib/tkextlib/blt/winop.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/blt.rb'
 
 module Tk::BLT
diff --git a/lib/tkextlib/bwidget.rb b/lib/tkextlib/bwidget.rb
index 0fa9c77e..bc1675b1 100644
--- a/lib/tkextlib/bwidget.rb
+++ b/lib/tkextlib/bwidget.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/bwidget/arrowbutton.rb b/lib/tkextlib/bwidget/arrowbutton.rb
index a9912450..23c190cc 100644
--- a/lib/tkextlib/bwidget/arrowbutton.rb
+++ b/lib/tkextlib/bwidget/arrowbutton.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/button'
 
diff --git a/lib/tkextlib/bwidget/bitmap.rb b/lib/tkextlib/bwidget/bitmap.rb
index 3acdfdf5..d3dd970b 100644
--- a/lib/tkextlib/bwidget/bitmap.rb
+++ b/lib/tkextlib/bwidget/bitmap.rb
@@ -3,7 +3,7 @@
 #  tkextlib/bwidget/bitmap.rb
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/image'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/button.rb b/lib/tkextlib/bwidget/button.rb
index ad880ebf..3a08def5 100644
--- a/lib/tkextlib/bwidget/button.rb
+++ b/lib/tkextlib/bwidget/button.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/button'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/buttonbox.rb b/lib/tkextlib/bwidget/buttonbox.rb
index 780a6a64..871a641e 100644
--- a/lib/tkextlib/bwidget/buttonbox.rb
+++ b/lib/tkextlib/bwidget/buttonbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/button'
 
diff --git a/lib/tkextlib/bwidget/combobox.rb b/lib/tkextlib/bwidget/combobox.rb
index b3de3186..ff4fb76a 100644
--- a/lib/tkextlib/bwidget/combobox.rb
+++ b/lib/tkextlib/bwidget/combobox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/entry'
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/listbox'
diff --git a/lib/tkextlib/bwidget/dialog.rb b/lib/tkextlib/bwidget/dialog.rb
index ed929cb8..fa30cb4a 100644
--- a/lib/tkextlib/bwidget/dialog.rb
+++ b/lib/tkextlib/bwidget/dialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/buttonbox'
diff --git a/lib/tkextlib/bwidget/dragsite.rb b/lib/tkextlib/bwidget/dragsite.rb
index 6c7ea449..e81458b2 100644
--- a/lib/tkextlib/bwidget/dragsite.rb
+++ b/lib/tkextlib/bwidget/dragsite.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 
 module Tk
diff --git a/lib/tkextlib/bwidget/dropsite.rb b/lib/tkextlib/bwidget/dropsite.rb
index e5eb7f4a..482f0caf 100644
--- a/lib/tkextlib/bwidget/dropsite.rb
+++ b/lib/tkextlib/bwidget/dropsite.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 
 module Tk
diff --git a/lib/tkextlib/bwidget/dynamichelp.rb b/lib/tkextlib/bwidget/dynamichelp.rb
index 7e7538fc..1c77330e 100644
--- a/lib/tkextlib/bwidget/dynamichelp.rb
+++ b/lib/tkextlib/bwidget/dynamichelp.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 
 module Tk
diff --git a/lib/tkextlib/bwidget/entry.rb b/lib/tkextlib/bwidget/entry.rb
index bfbe2f19..7eb2f16a 100644
--- a/lib/tkextlib/bwidget/entry.rb
+++ b/lib/tkextlib/bwidget/entry.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/entry'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/label.rb b/lib/tkextlib/bwidget/label.rb
index 9a0b73d9..27a5cd1c 100644
--- a/lib/tkextlib/bwidget/label.rb
+++ b/lib/tkextlib/bwidget/label.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/label'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/labelentry.rb b/lib/tkextlib/bwidget/labelentry.rb
index b2b68941..5bb7d91a 100644
--- a/lib/tkextlib/bwidget/labelentry.rb
+++ b/lib/tkextlib/bwidget/labelentry.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/entry'
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/labelframe'
diff --git a/lib/tkextlib/bwidget/labelframe.rb b/lib/tkextlib/bwidget/labelframe.rb
index 21c529c6..f5c9504f 100644
--- a/lib/tkextlib/bwidget/labelframe.rb
+++ b/lib/tkextlib/bwidget/labelframe.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/label'
diff --git a/lib/tkextlib/bwidget/listbox.rb b/lib/tkextlib/bwidget/listbox.rb
index cf3da2f9..4591fbaf 100644
--- a/lib/tkextlib/bwidget/listbox.rb
+++ b/lib/tkextlib/bwidget/listbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/canvas'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/mainframe.rb b/lib/tkextlib/bwidget/mainframe.rb
index ad097439..853c9e8b 100644
--- a/lib/tkextlib/bwidget/mainframe.rb
+++ b/lib/tkextlib/bwidget/mainframe.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/progressbar'
diff --git a/lib/tkextlib/bwidget/messagedlg.rb b/lib/tkextlib/bwidget/messagedlg.rb
index 69819360..cefef3f0 100644
--- a/lib/tkextlib/bwidget/messagedlg.rb
+++ b/lib/tkextlib/bwidget/messagedlg.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/dialog.rb'
 
diff --git a/lib/tkextlib/bwidget/notebook.rb b/lib/tkextlib/bwidget/notebook.rb
index ac29c9af..4a26b46d 100644
--- a/lib/tkextlib/bwidget/notebook.rb
+++ b/lib/tkextlib/bwidget/notebook.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/pagesmanager.rb b/lib/tkextlib/bwidget/pagesmanager.rb
index ce18ee0e..7b1e33a2 100644
--- a/lib/tkextlib/bwidget/pagesmanager.rb
+++ b/lib/tkextlib/bwidget/pagesmanager.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/panedwindow.rb b/lib/tkextlib/bwidget/panedwindow.rb
index 560c5634..167ca0d7 100644
--- a/lib/tkextlib/bwidget/panedwindow.rb
+++ b/lib/tkextlib/bwidget/panedwindow.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/panelframe.rb b/lib/tkextlib/bwidget/panelframe.rb
index 0e32f4e2..d2198dfa 100644
--- a/lib/tkextlib/bwidget/panelframe.rb
+++ b/lib/tkextlib/bwidget/panelframe.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/passwddlg.rb b/lib/tkextlib/bwidget/passwddlg.rb
index 9fe6fc6a..f8cafb1f 100644
--- a/lib/tkextlib/bwidget/passwddlg.rb
+++ b/lib/tkextlib/bwidget/passwddlg.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/messagedlg'
 
diff --git a/lib/tkextlib/bwidget/progressbar.rb b/lib/tkextlib/bwidget/progressbar.rb
index f06ddc16..4d322a6e 100644
--- a/lib/tkextlib/bwidget/progressbar.rb
+++ b/lib/tkextlib/bwidget/progressbar.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 
 module Tk
diff --git a/lib/tkextlib/bwidget/progressdlg.rb b/lib/tkextlib/bwidget/progressdlg.rb
index b378eaff..fcdba7c6 100644
--- a/lib/tkextlib/bwidget/progressdlg.rb
+++ b/lib/tkextlib/bwidget/progressdlg.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/variable'
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/progressbar'
diff --git a/lib/tkextlib/bwidget/scrollableframe.rb b/lib/tkextlib/bwidget/scrollableframe.rb
index 2b184746..ce170380 100644
--- a/lib/tkextlib/bwidget/scrollableframe.rb
+++ b/lib/tkextlib/bwidget/scrollableframe.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/scrolledwindow.rb b/lib/tkextlib/bwidget/scrolledwindow.rb
index 9bd5e875..8484a463 100644
--- a/lib/tkextlib/bwidget/scrolledwindow.rb
+++ b/lib/tkextlib/bwidget/scrolledwindow.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/scrollview.rb b/lib/tkextlib/bwidget/scrollview.rb
index d60fdca8..a92e4559 100644
--- a/lib/tkextlib/bwidget/scrollview.rb
+++ b/lib/tkextlib/bwidget/scrollview.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 
 module Tk
diff --git a/lib/tkextlib/bwidget/selectcolor.rb b/lib/tkextlib/bwidget/selectcolor.rb
index 85809b69..c8367f9c 100644
--- a/lib/tkextlib/bwidget/selectcolor.rb
+++ b/lib/tkextlib/bwidget/selectcolor.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/messagedlg'
 
diff --git a/lib/tkextlib/bwidget/selectfont.rb b/lib/tkextlib/bwidget/selectfont.rb
index 4f67a2e2..5a97ff00 100644
--- a/lib/tkextlib/bwidget/selectfont.rb
+++ b/lib/tkextlib/bwidget/selectfont.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/messagedlg'
 
diff --git a/lib/tkextlib/bwidget/separator.rb b/lib/tkextlib/bwidget/separator.rb
index 786c41c7..f65de93b 100644
--- a/lib/tkextlib/bwidget/separator.rb
+++ b/lib/tkextlib/bwidget/separator.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 
 module Tk
diff --git a/lib/tkextlib/bwidget/spinbox.rb b/lib/tkextlib/bwidget/spinbox.rb
index 2e967dd0..1e8369cc 100644
--- a/lib/tkextlib/bwidget/spinbox.rb
+++ b/lib/tkextlib/bwidget/spinbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 require 'tkextlib/bwidget/arrowbutton'
 require 'tkextlib/bwidget/entry'
diff --git a/lib/tkextlib/bwidget/statusbar.rb b/lib/tkextlib/bwidget/statusbar.rb
index 28fd6fab..9c528f75 100644
--- a/lib/tkextlib/bwidget/statusbar.rb
+++ b/lib/tkextlib/bwidget/statusbar.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/titleframe.rb b/lib/tkextlib/bwidget/titleframe.rb
index eca62239..95cd9be1 100644
--- a/lib/tkextlib/bwidget/titleframe.rb
+++ b/lib/tkextlib/bwidget/titleframe.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/frame'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/tree.rb b/lib/tkextlib/bwidget/tree.rb
index d0b2f473..56c71718 100644
--- a/lib/tkextlib/bwidget/tree.rb
+++ b/lib/tkextlib/bwidget/tree.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/canvas'
 require 'tkextlib/bwidget.rb'
 
diff --git a/lib/tkextlib/bwidget/widget.rb b/lib/tkextlib/bwidget/widget.rb
index ab08826b..e88a8682 100644
--- a/lib/tkextlib/bwidget/widget.rb
+++ b/lib/tkextlib/bwidget/widget.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/bwidget.rb'
 
 module Tk
diff --git a/lib/tkextlib/itcl/incr_tcl.rb b/lib/tkextlib/itcl/incr_tcl.rb
index 63aacbb8..86f5c34a 100644
--- a/lib/tkextlib/itcl/incr_tcl.rb
+++ b/lib/tkextlib/itcl/incr_tcl.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script
 require 'tkextlib/itcl.rb'
diff --git a/lib/tkextlib/itk/incr_tk.rb b/lib/tkextlib/itk/incr_tk.rb
index c51a45a1..281c4d9e 100644
--- a/lib/tkextlib/itk/incr_tk.rb
+++ b/lib/tkextlib/itk/incr_tk.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/menuspec'
 require 'tkextlib/itcl.rb'
 
diff --git a/lib/tkextlib/iwidgets.rb b/lib/tkextlib/iwidgets.rb
index 2ebd9d99..157c9e23 100644
--- a/lib/tkextlib/iwidgets.rb
+++ b/lib/tkextlib/iwidgets.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/itcl'
 require 'tkextlib/itk'
 
diff --git a/lib/tkextlib/iwidgets/buttonbox.rb b/lib/tkextlib/iwidgets/buttonbox.rb
index 0357114c..bc440310 100644
--- a/lib/tkextlib/iwidgets/buttonbox.rb
+++ b/lib/tkextlib/iwidgets/buttonbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/calendar.rb b/lib/tkextlib/iwidgets/calendar.rb
index de9909f9..9b160cce 100644
--- a/lib/tkextlib/iwidgets/calendar.rb
+++ b/lib/tkextlib/iwidgets/calendar.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/canvasprintbox.rb b/lib/tkextlib/iwidgets/canvasprintbox.rb
index 3fe489dc..1b4afe57 100644
--- a/lib/tkextlib/iwidgets/canvasprintbox.rb
+++ b/lib/tkextlib/iwidgets/canvasprintbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/canvasprintdialog.rb b/lib/tkextlib/iwidgets/canvasprintdialog.rb
index 4b5fdf5d..870f0ebd 100644
--- a/lib/tkextlib/iwidgets/canvasprintdialog.rb
+++ b/lib/tkextlib/iwidgets/canvasprintdialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/checkbox.rb b/lib/tkextlib/iwidgets/checkbox.rb
index 4589fe7f..c5b5f418 100644
--- a/lib/tkextlib/iwidgets/checkbox.rb
+++ b/lib/tkextlib/iwidgets/checkbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/combobox.rb b/lib/tkextlib/iwidgets/combobox.rb
index d4c05b5b..ca9e4d0d 100644
--- a/lib/tkextlib/iwidgets/combobox.rb
+++ b/lib/tkextlib/iwidgets/combobox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/dateentry.rb b/lib/tkextlib/iwidgets/dateentry.rb
index e135b89e..c1759385 100644
--- a/lib/tkextlib/iwidgets/dateentry.rb
+++ b/lib/tkextlib/iwidgets/dateentry.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/datefield.rb b/lib/tkextlib/iwidgets/datefield.rb
index 35977e1f..f9c96344 100644
--- a/lib/tkextlib/iwidgets/datefield.rb
+++ b/lib/tkextlib/iwidgets/datefield.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/dialog.rb b/lib/tkextlib/iwidgets/dialog.rb
index edefba18..c7587915 100644
--- a/lib/tkextlib/iwidgets/dialog.rb
+++ b/lib/tkextlib/iwidgets/dialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/dialogshell.rb b/lib/tkextlib/iwidgets/dialogshell.rb
index 009fc61c..f6bb35f2 100644
--- a/lib/tkextlib/iwidgets/dialogshell.rb
+++ b/lib/tkextlib/iwidgets/dialogshell.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/disjointlistbox.rb b/lib/tkextlib/iwidgets/disjointlistbox.rb
index 3ae3f319..d26ff8f6 100644
--- a/lib/tkextlib/iwidgets/disjointlistbox.rb
+++ b/lib/tkextlib/iwidgets/disjointlistbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/entryfield.rb b/lib/tkextlib/iwidgets/entryfield.rb
index f91e5d6d..33171663 100644
--- a/lib/tkextlib/iwidgets/entryfield.rb
+++ b/lib/tkextlib/iwidgets/entryfield.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/extbutton.rb b/lib/tkextlib/iwidgets/extbutton.rb
index 3c77c2ec..d10312a8 100644
--- a/lib/tkextlib/iwidgets/extbutton.rb
+++ b/lib/tkextlib/iwidgets/extbutton.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/extfileselectionbox.rb b/lib/tkextlib/iwidgets/extfileselectionbox.rb
index 45817713..45d1cc4f 100644
--- a/lib/tkextlib/iwidgets/extfileselectionbox.rb
+++ b/lib/tkextlib/iwidgets/extfileselectionbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/extfileselectiondialog.rb b/lib/tkextlib/iwidgets/extfileselectiondialog.rb
index 32a06cc4..299e89ca 100644
--- a/lib/tkextlib/iwidgets/extfileselectiondialog.rb
+++ b/lib/tkextlib/iwidgets/extfileselectiondialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/feedback.rb b/lib/tkextlib/iwidgets/feedback.rb
index e52d60ed..44cdbb6d 100644
--- a/lib/tkextlib/iwidgets/feedback.rb
+++ b/lib/tkextlib/iwidgets/feedback.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/fileselectionbox.rb b/lib/tkextlib/iwidgets/fileselectionbox.rb
index 3811f2b5..9bc3ec0a 100644
--- a/lib/tkextlib/iwidgets/fileselectionbox.rb
+++ b/lib/tkextlib/iwidgets/fileselectionbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/fileselectiondialog.rb b/lib/tkextlib/iwidgets/fileselectiondialog.rb
index 8d368486..45b9ee09 100644
--- a/lib/tkextlib/iwidgets/fileselectiondialog.rb
+++ b/lib/tkextlib/iwidgets/fileselectiondialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/finddialog.rb b/lib/tkextlib/iwidgets/finddialog.rb
index ff0b5873..7a6efe47 100644
--- a/lib/tkextlib/iwidgets/finddialog.rb
+++ b/lib/tkextlib/iwidgets/finddialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/hierarchy.rb b/lib/tkextlib/iwidgets/hierarchy.rb
index af999732..81d4cb6a 100644
--- a/lib/tkextlib/iwidgets/hierarchy.rb
+++ b/lib/tkextlib/iwidgets/hierarchy.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/text'
 require 'tkextlib/iwidgets.rb'
 
diff --git a/lib/tkextlib/iwidgets/hyperhelp.rb b/lib/tkextlib/iwidgets/hyperhelp.rb
index 1650c7af..50878fcb 100644
--- a/lib/tkextlib/iwidgets/hyperhelp.rb
+++ b/lib/tkextlib/iwidgets/hyperhelp.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/labeledframe.rb b/lib/tkextlib/iwidgets/labeledframe.rb
index f4f37865..e236e6cc 100644
--- a/lib/tkextlib/iwidgets/labeledframe.rb
+++ b/lib/tkextlib/iwidgets/labeledframe.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/labeledwidget.rb b/lib/tkextlib/iwidgets/labeledwidget.rb
index fe3c0313..e10b0d5d 100644
--- a/lib/tkextlib/iwidgets/labeledwidget.rb
+++ b/lib/tkextlib/iwidgets/labeledwidget.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/mainwindow.rb b/lib/tkextlib/iwidgets/mainwindow.rb
index 34c9eb52..905249d7 100644
--- a/lib/tkextlib/iwidgets/mainwindow.rb
+++ b/lib/tkextlib/iwidgets/mainwindow.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/menubar.rb b/lib/tkextlib/iwidgets/menubar.rb
index f7295112..e36a033b 100644
--- a/lib/tkextlib/iwidgets/menubar.rb
+++ b/lib/tkextlib/iwidgets/menubar.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/messagebox.rb b/lib/tkextlib/iwidgets/messagebox.rb
index a4c28228..a606f33b 100644
--- a/lib/tkextlib/iwidgets/messagebox.rb
+++ b/lib/tkextlib/iwidgets/messagebox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/messagedialog.rb b/lib/tkextlib/iwidgets/messagedialog.rb
index 2fb4533b..f812d477 100644
--- a/lib/tkextlib/iwidgets/messagedialog.rb
+++ b/lib/tkextlib/iwidgets/messagedialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/notebook.rb b/lib/tkextlib/iwidgets/notebook.rb
index b29aa595..066606d1 100644
--- a/lib/tkextlib/iwidgets/notebook.rb
+++ b/lib/tkextlib/iwidgets/notebook.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/optionmenu.rb b/lib/tkextlib/iwidgets/optionmenu.rb
index 642db11f..764e278a 100644
--- a/lib/tkextlib/iwidgets/optionmenu.rb
+++ b/lib/tkextlib/iwidgets/optionmenu.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/panedwindow.rb b/lib/tkextlib/iwidgets/panedwindow.rb
index 5f17bdd8..34d34821 100644
--- a/lib/tkextlib/iwidgets/panedwindow.rb
+++ b/lib/tkextlib/iwidgets/panedwindow.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/promptdialog.rb b/lib/tkextlib/iwidgets/promptdialog.rb
index 228f8c45..a6eb5db7 100644
--- a/lib/tkextlib/iwidgets/promptdialog.rb
+++ b/lib/tkextlib/iwidgets/promptdialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/pushbutton.rb b/lib/tkextlib/iwidgets/pushbutton.rb
index cec3563d..3a0ddadb 100644
--- a/lib/tkextlib/iwidgets/pushbutton.rb
+++ b/lib/tkextlib/iwidgets/pushbutton.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/radiobox.rb b/lib/tkextlib/iwidgets/radiobox.rb
index 1c9e9f8d..d33151cd 100644
--- a/lib/tkextlib/iwidgets/radiobox.rb
+++ b/lib/tkextlib/iwidgets/radiobox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/scopedobject.rb b/lib/tkextlib/iwidgets/scopedobject.rb
index e17fee98..bd14da2f 100644
--- a/lib/tkextlib/iwidgets/scopedobject.rb
+++ b/lib/tkextlib/iwidgets/scopedobject.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/scrolledcanvas.rb b/lib/tkextlib/iwidgets/scrolledcanvas.rb
index d345552d..313916a2 100644
--- a/lib/tkextlib/iwidgets/scrolledcanvas.rb
+++ b/lib/tkextlib/iwidgets/scrolledcanvas.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/canvas'
 require 'tkextlib/iwidgets.rb'
 
diff --git a/lib/tkextlib/iwidgets/scrolledframe.rb b/lib/tkextlib/iwidgets/scrolledframe.rb
index f105576f..532d9a2d 100644
--- a/lib/tkextlib/iwidgets/scrolledframe.rb
+++ b/lib/tkextlib/iwidgets/scrolledframe.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/scrolledhtml.rb b/lib/tkextlib/iwidgets/scrolledhtml.rb
index 4ee5abe9..48387fd3 100644
--- a/lib/tkextlib/iwidgets/scrolledhtml.rb
+++ b/lib/tkextlib/iwidgets/scrolledhtml.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/scrolledlistbox.rb b/lib/tkextlib/iwidgets/scrolledlistbox.rb
index 94949c55..3c644c2e 100644
--- a/lib/tkextlib/iwidgets/scrolledlistbox.rb
+++ b/lib/tkextlib/iwidgets/scrolledlistbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/listbox'
 require 'tkextlib/iwidgets.rb'
 
diff --git a/lib/tkextlib/iwidgets/scrolledtext.rb b/lib/tkextlib/iwidgets/scrolledtext.rb
index 1e289898..8d5f199a 100644
--- a/lib/tkextlib/iwidgets/scrolledtext.rb
+++ b/lib/tkextlib/iwidgets/scrolledtext.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/text'
 require 'tkextlib/iwidgets.rb'
 
diff --git a/lib/tkextlib/iwidgets/scrolledwidget.rb b/lib/tkextlib/iwidgets/scrolledwidget.rb
index a8b4e5a2..528f04fa 100644
--- a/lib/tkextlib/iwidgets/scrolledwidget.rb
+++ b/lib/tkextlib/iwidgets/scrolledwidget.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/selectionbox.rb b/lib/tkextlib/iwidgets/selectionbox.rb
index 333f68e3..1e8b7140 100644
--- a/lib/tkextlib/iwidgets/selectionbox.rb
+++ b/lib/tkextlib/iwidgets/selectionbox.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/selectiondialog.rb b/lib/tkextlib/iwidgets/selectiondialog.rb
index 0d0b94e8..b2dd300e 100644
--- a/lib/tkextlib/iwidgets/selectiondialog.rb
+++ b/lib/tkextlib/iwidgets/selectiondialog.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/shell.rb b/lib/tkextlib/iwidgets/shell.rb
index b3f8dc7f..48a17cf8 100644
--- a/lib/tkextlib/iwidgets/shell.rb
+++ b/lib/tkextlib/iwidgets/shell.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/spindate.rb b/lib/tkextlib/iwidgets/spindate.rb
index 8860348c..450b7f50 100644
--- a/lib/tkextlib/iwidgets/spindate.rb
+++ b/lib/tkextlib/iwidgets/spindate.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/spinint.rb b/lib/tkextlib/iwidgets/spinint.rb
index 85736d09..415cd0cf 100644
--- a/lib/tkextlib/iwidgets/spinint.rb
+++ b/lib/tkextlib/iwidgets/spinint.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/spinner.rb b/lib/tkextlib/iwidgets/spinner.rb
index 9ced1c08..b1e4a98a 100644
--- a/lib/tkextlib/iwidgets/spinner.rb
+++ b/lib/tkextlib/iwidgets/spinner.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/spintime.rb b/lib/tkextlib/iwidgets/spintime.rb
index 9b280ec0..4b82e224 100644
--- a/lib/tkextlib/iwidgets/spintime.rb
+++ b/lib/tkextlib/iwidgets/spintime.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/tabnotebook.rb b/lib/tkextlib/iwidgets/tabnotebook.rb
index afd5d867..b4daff14 100644
--- a/lib/tkextlib/iwidgets/tabnotebook.rb
+++ b/lib/tkextlib/iwidgets/tabnotebook.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/tabset.rb b/lib/tkextlib/iwidgets/tabset.rb
index 3b4c141f..003e24b8 100644
--- a/lib/tkextlib/iwidgets/tabset.rb
+++ b/lib/tkextlib/iwidgets/tabset.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/timeentry.rb b/lib/tkextlib/iwidgets/timeentry.rb
index d4078e6e..1ffd25d8 100644
--- a/lib/tkextlib/iwidgets/timeentry.rb
+++ b/lib/tkextlib/iwidgets/timeentry.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/timefield.rb b/lib/tkextlib/iwidgets/timefield.rb
index d759a076..b163180c 100644
--- a/lib/tkextlib/iwidgets/timefield.rb
+++ b/lib/tkextlib/iwidgets/timefield.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/toolbar.rb b/lib/tkextlib/iwidgets/toolbar.rb
index 7f2a54b8..7c92a5e0 100644
--- a/lib/tkextlib/iwidgets/toolbar.rb
+++ b/lib/tkextlib/iwidgets/toolbar.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/iwidgets/watch.rb b/lib/tkextlib/iwidgets/watch.rb
index c6e862b3..5339fa06 100644
--- a/lib/tkextlib/iwidgets/watch.rb
+++ b/lib/tkextlib/iwidgets/watch.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/iwidgets.rb'
 
 module Tk
diff --git a/lib/tkextlib/pkg_checker.rb b/lib/tkextlib/pkg_checker.rb
index 868a813d..b9feb912 100755
--- a/lib/tkextlib/pkg_checker.rb
+++ b/lib/tkextlib/pkg_checker.rb
@@ -5,7 +5,7 @@
 #
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 TkRoot.new.withdraw  # hide root window
 
diff --git a/lib/tkextlib/tcllib.rb b/lib/tkextlib/tcllib.rb
index 75d250cb..b0774ec8 100644
--- a/lib/tkextlib/tcllib.rb
+++ b/lib/tkextlib/tcllib.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tcllib/autoscroll.rb b/lib/tkextlib/tcllib/autoscroll.rb
index ae45b847..cf10fdb6 100644
--- a/lib/tkextlib/tcllib/autoscroll.rb
+++ b/lib/tkextlib/tcllib/autoscroll.rb
@@ -25,7 +25,7 @@
 # subcommand is not used then this package will have no effect.
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/scrollbar'
 require 'tkextlib/tcllib.rb'
 
diff --git a/lib/tkextlib/tcllib/calendar.rb b/lib/tkextlib/tcllib/calendar.rb
index fa53603d..8f6b99d3 100644
--- a/lib/tkextlib/tcllib/calendar.rb
+++ b/lib/tkextlib/tcllib/calendar.rb
@@ -7,7 +7,7 @@
 #   * calendar widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::calendar', '0.9')
diff --git a/lib/tkextlib/tcllib/canvas_sqmap.rb b/lib/tkextlib/tcllib/canvas_sqmap.rb
index f3d1847d..5c40e20f 100644
--- a/lib/tkextlib/tcllib/canvas_sqmap.rb
+++ b/lib/tkextlib/tcllib/canvas_sqmap.rb
@@ -7,7 +7,7 @@
 #   *
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::canvas_sqmap', '0.2')
diff --git a/lib/tkextlib/tcllib/canvas_zoom.rb b/lib/tkextlib/tcllib/canvas_zoom.rb
index bb8b04a1..8c5f7ef2 100644
--- a/lib/tkextlib/tcllib/canvas_zoom.rb
+++ b/lib/tkextlib/tcllib/canvas_zoom.rb
@@ -7,7 +7,7 @@
 #   *
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::canvas_zoom', '0.1')
diff --git a/lib/tkextlib/tcllib/chatwidget.rb b/lib/tkextlib/tcllib/chatwidget.rb
index 34a8ca50..183dec50 100644
--- a/lib/tkextlib/tcllib/chatwidget.rb
+++ b/lib/tkextlib/tcllib/chatwidget.rb
@@ -8,7 +8,7 @@
 #                  chat room or irc channel information
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('chatwidget', '1.1.0')
diff --git a/lib/tkextlib/tcllib/crosshair.rb b/lib/tkextlib/tcllib/crosshair.rb
index fba6d99c..6c2f2c31 100644
--- a/lib/tkextlib/tcllib/crosshair.rb
+++ b/lib/tkextlib/tcllib/crosshair.rb
@@ -7,7 +7,7 @@
 #   * Crosshairs for Tk canvas
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('crosshair', '1.0.2')
diff --git a/lib/tkextlib/tcllib/ctext.rb b/lib/tkextlib/tcllib/ctext.rb
index e9ac70e6..1001a5f0 100644
--- a/lib/tkextlib/tcllib/ctext.rb
+++ b/lib/tkextlib/tcllib/ctext.rb
@@ -7,7 +7,7 @@
 #   * Overloads the text widget and provides new commands
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/text'
 require 'tkextlib/tcllib.rb'
 
diff --git a/lib/tkextlib/tcllib/cursor.rb b/lib/tkextlib/tcllib/cursor.rb
index 4c93cfc0..64faebc1 100644
--- a/lib/tkextlib/tcllib/cursor.rb
+++ b/lib/tkextlib/tcllib/cursor.rb
@@ -7,7 +7,7 @@
 #   * Procedures to handle CURSOR data
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 module Tk
diff --git a/lib/tkextlib/tcllib/dateentry.rb b/lib/tkextlib/tcllib/dateentry.rb
index 81b38515..db8d72ed 100644
--- a/lib/tkextlib/tcllib/dateentry.rb
+++ b/lib/tkextlib/tcllib/dateentry.rb
@@ -7,7 +7,7 @@
 #   * dateentry widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::dateentry', '0.91')
diff --git a/lib/tkextlib/tcllib/datefield.rb b/lib/tkextlib/tcllib/datefield.rb
index ceed1260..f170cac5 100644
--- a/lib/tkextlib/tcllib/datefield.rb
+++ b/lib/tkextlib/tcllib/datefield.rb
@@ -16,7 +16,7 @@
 # bindings. This means all the command and options for an entry widget apply
 # equally here.
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/entry'
 require 'tkextlib/tcllib.rb'
 
diff --git a/lib/tkextlib/tcllib/diagrams.rb b/lib/tkextlib/tcllib/diagrams.rb
index 7fb7b3c7..b1d7d774 100644
--- a/lib/tkextlib/tcllib/diagrams.rb
+++ b/lib/tkextlib/tcllib/diagrams.rb
@@ -7,7 +7,7 @@
 #   * Draw diagrams
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('Diagrams', '0.3')
diff --git a/lib/tkextlib/tcllib/dialog.rb b/lib/tkextlib/tcllib/dialog.rb
index 46fd06f1..c62b8796 100644
--- a/lib/tkextlib/tcllib/dialog.rb
+++ b/lib/tkextlib/tcllib/dialog.rb
@@ -7,7 +7,7 @@
 #   * Generic dialog widget (themed)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::dialog', '1.2')
diff --git a/lib/tkextlib/tcllib/getstring.rb b/lib/tkextlib/tcllib/getstring.rb
index 72d9c5f5..c8f21859 100644
--- a/lib/tkextlib/tcllib/getstring.rb
+++ b/lib/tkextlib/tcllib/getstring.rb
@@ -7,7 +7,7 @@
 #   * A dialog which consists of an Entry, OK, and Cancel buttons.
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/entry'
 require 'tkextlib/tcllib.rb'
 
diff --git a/lib/tkextlib/tcllib/history.rb b/lib/tkextlib/tcllib/history.rb
index a1b92726..942d8269 100644
--- a/lib/tkextlib/tcllib/history.rb
+++ b/lib/tkextlib/tcllib/history.rb
@@ -7,7 +7,7 @@
 #   * Provides a history for Entry widgets
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/entry'
 require 'tkextlib/tcllib.rb'
 
diff --git a/lib/tkextlib/tcllib/ico.rb b/lib/tkextlib/tcllib/ico.rb
index 8814205f..73ec138a 100644
--- a/lib/tkextlib/tcllib/ico.rb
+++ b/lib/tkextlib/tcllib/ico.rb
@@ -7,7 +7,7 @@
 #   * Reading and writing windows icons
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/image'
 #require 'tkextlib/tcllib.rb'
 
diff --git a/lib/tkextlib/tcllib/ip_entry.rb b/lib/tkextlib/tcllib/ip_entry.rb
index 6bbd8442..04866bc4 100644
--- a/lib/tkextlib/tcllib/ip_entry.rb
+++ b/lib/tkextlib/tcllib/ip_entry.rb
@@ -11,7 +11,7 @@
 # This package provides a widget for the entering of a IP address.
 # It guarantees a valid address at all times.
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('ipentry', '0.1')
diff --git a/lib/tkextlib/tcllib/khim.rb b/lib/tkextlib/tcllib/khim.rb
index e79f26b3..915f9880 100644
--- a/lib/tkextlib/tcllib/khim.rb
+++ b/lib/tkextlib/tcllib/khim.rb
@@ -7,7 +7,7 @@
 #   * Kevin's Hacky Input Method
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('khim', '1.0')
diff --git a/lib/tkextlib/tcllib/menuentry.rb b/lib/tkextlib/tcllib/menuentry.rb
index 51521499..09ccb4cd 100644
--- a/lib/tkextlib/tcllib/menuentry.rb
+++ b/lib/tkextlib/tcllib/menuentry.rb
@@ -7,7 +7,7 @@
 #   * menuentry widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::menuentry', '1.0')
diff --git a/lib/tkextlib/tcllib/ntext.rb b/lib/tkextlib/tcllib/ntext.rb
index 2d0c2082..9cd3b504 100644
--- a/lib/tkextlib/tcllib/ntext.rb
+++ b/lib/tkextlib/tcllib/ntext.rb
@@ -7,7 +7,7 @@
 #   * Ntext bindtag
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('ntext', '0.81')
diff --git a/lib/tkextlib/tcllib/panelframe.rb b/lib/tkextlib/tcllib/panelframe.rb
index 678d5d0a..44df62cb 100644
--- a/lib/tkextlib/tcllib/panelframe.rb
+++ b/lib/tkextlib/tcllib/panelframe.rb
@@ -7,7 +7,7 @@
 #   * Create PanelFrame widgets.
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::panelframe', '1.0')
diff --git a/lib/tkextlib/tcllib/plotchart.rb b/lib/tkextlib/tcllib/plotchart.rb
index 2716c6e8..9a251abe 100644
--- a/lib/tkextlib/tcllib/plotchart.rb
+++ b/lib/tkextlib/tcllib/plotchart.rb
@@ -58,7 +58,7 @@
 #    * 3D plots (both for displaying surfaces and 3D bars)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('Plotchart', '0.9')
diff --git a/lib/tkextlib/tcllib/ruler.rb b/lib/tkextlib/tcllib/ruler.rb
index f57b0b0e..5d1db457 100644
--- a/lib/tkextlib/tcllib/ruler.rb
+++ b/lib/tkextlib/tcllib/ruler.rb
@@ -7,7 +7,7 @@
 #   * ruler widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::ruler', '1.0')
diff --git a/lib/tkextlib/tcllib/screenruler.rb b/lib/tkextlib/tcllib/screenruler.rb
index 64dd4f4c..04c56190 100644
--- a/lib/tkextlib/tcllib/screenruler.rb
+++ b/lib/tkextlib/tcllib/screenruler.rb
@@ -7,7 +7,7 @@
 #   * screenruler dialog
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::screenruler', '1.1')
diff --git a/lib/tkextlib/tcllib/scrolledwindow.rb b/lib/tkextlib/tcllib/scrolledwindow.rb
index 380d301a..e00b79c9 100644
--- a/lib/tkextlib/tcllib/scrolledwindow.rb
+++ b/lib/tkextlib/tcllib/scrolledwindow.rb
@@ -7,7 +7,7 @@
 #   * scrolledwindow widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::scrolledwindow', '1.2')
diff --git a/lib/tkextlib/tcllib/scrollwin.rb b/lib/tkextlib/tcllib/scrollwin.rb
index f9bf1277..a4a0aca5 100644
--- a/lib/tkextlib/tcllib/scrollwin.rb
+++ b/lib/tkextlib/tcllib/scrollwin.rb
@@ -7,7 +7,7 @@
 #   * Scrolled widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::scrolledwindow', '1.0')
diff --git a/lib/tkextlib/tcllib/statusbar.rb b/lib/tkextlib/tcllib/statusbar.rb
index b2c8f54b..181de487 100644
--- a/lib/tkextlib/tcllib/statusbar.rb
+++ b/lib/tkextlib/tcllib/statusbar.rb
@@ -7,7 +7,7 @@
 #   * statusbar widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::statusbar', '1.2')
diff --git a/lib/tkextlib/tcllib/style.rb b/lib/tkextlib/tcllib/style.rb
index f3b66b78..3d8307f7 100644
--- a/lib/tkextlib/tcllib/style.rb
+++ b/lib/tkextlib/tcllib/style.rb
@@ -7,7 +7,7 @@
 #   * select and use some 'style' of option (resource) DB
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 module Tk::Tcllib
diff --git a/lib/tkextlib/tcllib/superframe.rb b/lib/tkextlib/tcllib/superframe.rb
index 776ad7c1..ccfcced9 100644
--- a/lib/tkextlib/tcllib/superframe.rb
+++ b/lib/tkextlib/tcllib/superframe.rb
@@ -7,7 +7,7 @@
 #   * Superframe widget - enhanced labelframe widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::superframe', '1.0')
diff --git a/lib/tkextlib/tcllib/swaplist.rb b/lib/tkextlib/tcllib/swaplist.rb
index 7eaed8db..4849aa25 100644
--- a/lib/tkextlib/tcllib/swaplist.rb
+++ b/lib/tkextlib/tcllib/swaplist.rb
@@ -7,7 +7,7 @@
 #   * A dialog which allows a user to move options between two lists
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('swaplist', '0.1')
diff --git a/lib/tkextlib/tcllib/tablelist.rb b/lib/tkextlib/tcllib/tablelist.rb
index 7cfe2faf..140d7b76 100644
--- a/lib/tkextlib/tcllib/tablelist.rb
+++ b/lib/tkextlib/tcllib/tablelist.rb
@@ -6,7 +6,7 @@
 #   * Part of tcllib extension
 #   * A multi-column listbox
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # check Tile extension :: If already loaded, use tablelist_tile.
diff --git a/lib/tkextlib/tcllib/tablelist_tile.rb b/lib/tkextlib/tcllib/tablelist_tile.rb
index d467e549..c1d23ae0 100644
--- a/lib/tkextlib/tcllib/tablelist_tile.rb
+++ b/lib/tkextlib/tcllib/tablelist_tile.rb
@@ -6,7 +6,7 @@
 #   * Part of tcllib extension
 #   * A multi-column listbox
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('tablelist_tile', '4.2')
diff --git a/lib/tkextlib/tcllib/tkpiechart.rb b/lib/tkextlib/tcllib/tkpiechart.rb
index 2fc75a68..bc46133d 100644
--- a/lib/tkextlib/tcllib/tkpiechart.rb
+++ b/lib/tkextlib/tcllib/tkpiechart.rb
@@ -7,7 +7,7 @@
 #   * Create 2D or 3D pies with labels in Tcl canvases
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/canvas'
 require 'tkextlib/tcllib.rb'
 
diff --git a/lib/tkextlib/tcllib/toolbar.rb b/lib/tkextlib/tcllib/toolbar.rb
index a771548d..e81e1074 100644
--- a/lib/tkextlib/tcllib/toolbar.rb
+++ b/lib/tkextlib/tcllib/toolbar.rb
@@ -7,7 +7,7 @@
 #   * toolbar widget
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::toolbar', '1.2')
diff --git a/lib/tkextlib/tcllib/tooltip.rb b/lib/tkextlib/tcllib/tooltip.rb
index 4a70ec57..56e2d2cb 100644
--- a/lib/tkextlib/tcllib/tooltip.rb
+++ b/lib/tkextlib/tcllib/tooltip.rb
@@ -8,7 +8,7 @@
 #     mouse hovers over a widget.
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('tooltip', '1.1')
diff --git a/lib/tkextlib/tcllib/validator.rb b/lib/tkextlib/tcllib/validator.rb
index 4586dd04..5c4fe729 100644
--- a/lib/tkextlib/tcllib/validator.rb
+++ b/lib/tkextlib/tcllib/validator.rb
@@ -7,7 +7,7 @@
 #   * Provides a unified validation API
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget::validator', '0.1')
diff --git a/lib/tkextlib/tcllib/widget.rb b/lib/tkextlib/tcllib/widget.rb
index b8d5070b..b72aae57 100644
--- a/lib/tkextlib/tcllib/widget.rb
+++ b/lib/tkextlib/tcllib/widget.rb
@@ -7,7 +7,7 @@
 #   * megawidget package that uses snit as the object system (snidgets)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tcllib.rb'
 
 # TkPackage.require('widget', '3.0')
diff --git a/lib/tkextlib/tclx/tclx.rb b/lib/tkextlib/tclx/tclx.rb
index 7360a854..ce197b6e 100644
--- a/lib/tkextlib/tclx/tclx.rb
+++ b/lib/tkextlib/tclx/tclx.rb
@@ -3,7 +3,7 @@
 #  tclx/tclx.rb
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tile.rb b/lib/tkextlib/tile.rb
index 29feebeb..ad24b06e 100644
--- a/lib/tkextlib/tile.rb
+++ b/lib/tkextlib/tile.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/ttk_selector'
 
 # call setup script for general 'tkextlib' libraries
diff --git a/lib/tkextlib/tile/dialog.rb b/lib/tkextlib/tile/dialog.rb
index d81851bb..8159f128 100644
--- a/lib/tkextlib/tile/dialog.rb
+++ b/lib/tkextlib/tile/dialog.rb
@@ -3,7 +3,7 @@
 #  ttk::dialog  (tile-0.7+)
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/sizegrip.rb b/lib/tkextlib/tile/sizegrip.rb
index 4b9510ae..347927b4 100644
--- a/lib/tkextlib/tile/sizegrip.rb
+++ b/lib/tkextlib/tile/sizegrip.rb
@@ -3,7 +3,7 @@
 #  ttk::sizegrip widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/style.rb b/lib/tkextlib/tile/style.rb
index 8b022a4f..c6120558 100644
--- a/lib/tkextlib/tile/style.rb
+++ b/lib/tkextlib/tile/style.rb
@@ -3,7 +3,7 @@
 #  style commands
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tbutton.rb b/lib/tkextlib/tile/tbutton.rb
index 217da62e..17163e9c 100644
--- a/lib/tkextlib/tile/tbutton.rb
+++ b/lib/tkextlib/tile/tbutton.rb
@@ -3,7 +3,7 @@
 #  tbutton widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tcheckbutton.rb b/lib/tkextlib/tile/tcheckbutton.rb
index c0e4d4b5..3dd24ffc 100644
--- a/lib/tkextlib/tile/tcheckbutton.rb
+++ b/lib/tkextlib/tile/tcheckbutton.rb
@@ -3,7 +3,7 @@
 #  tcheckbutton widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tcombobox.rb b/lib/tkextlib/tile/tcombobox.rb
index f2dec21f..30598aab 100644
--- a/lib/tkextlib/tile/tcombobox.rb
+++ b/lib/tkextlib/tile/tcombobox.rb
@@ -3,7 +3,7 @@
 #  tcombobox widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tentry.rb b/lib/tkextlib/tile/tentry.rb
index 5d577a65..5d738b06 100644
--- a/lib/tkextlib/tile/tentry.rb
+++ b/lib/tkextlib/tile/tentry.rb
@@ -3,7 +3,7 @@
 #  tentry widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tframe.rb b/lib/tkextlib/tile/tframe.rb
index 5ebf19d1..66ad5c85 100644
--- a/lib/tkextlib/tile/tframe.rb
+++ b/lib/tkextlib/tile/tframe.rb
@@ -3,7 +3,7 @@
 #  tframe widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tlabel.rb b/lib/tkextlib/tile/tlabel.rb
index 3dddeafc..95560d44 100644
--- a/lib/tkextlib/tile/tlabel.rb
+++ b/lib/tkextlib/tile/tlabel.rb
@@ -3,7 +3,7 @@
 #  tlabel widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tlabelframe.rb b/lib/tkextlib/tile/tlabelframe.rb
index d512f31f..8fa813a5 100644
--- a/lib/tkextlib/tile/tlabelframe.rb
+++ b/lib/tkextlib/tile/tlabelframe.rb
@@ -3,7 +3,7 @@
 #  tlabelframe widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tmenubutton.rb b/lib/tkextlib/tile/tmenubutton.rb
index a22ebaec..a773c1db 100644
--- a/lib/tkextlib/tile/tmenubutton.rb
+++ b/lib/tkextlib/tile/tmenubutton.rb
@@ -3,7 +3,7 @@
 #  tmenubutton widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tnotebook.rb b/lib/tkextlib/tile/tnotebook.rb
index 4089dcfd..3beb8f0b 100644
--- a/lib/tkextlib/tile/tnotebook.rb
+++ b/lib/tkextlib/tile/tnotebook.rb
@@ -3,7 +3,7 @@
 #  tnotebook widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tpaned.rb b/lib/tkextlib/tile/tpaned.rb
index f51a1f3b..25599227 100644
--- a/lib/tkextlib/tile/tpaned.rb
+++ b/lib/tkextlib/tile/tpaned.rb
@@ -3,7 +3,7 @@
 #  tpaned widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tprogressbar.rb b/lib/tkextlib/tile/tprogressbar.rb
index 446e053e..bf41d3d5 100644
--- a/lib/tkextlib/tile/tprogressbar.rb
+++ b/lib/tkextlib/tile/tprogressbar.rb
@@ -3,7 +3,7 @@
 #  tprogressbar widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tradiobutton.rb b/lib/tkextlib/tile/tradiobutton.rb
index 202de1eb..8ac9207a 100644
--- a/lib/tkextlib/tile/tradiobutton.rb
+++ b/lib/tkextlib/tile/tradiobutton.rb
@@ -3,7 +3,7 @@
 #  tradiobutton widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/treeview.rb b/lib/tkextlib/tile/treeview.rb
index 1c92db5c..d23c36d8 100644
--- a/lib/tkextlib/tile/treeview.rb
+++ b/lib/tkextlib/tile/treeview.rb
@@ -3,7 +3,7 @@
 #  treeview widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tscale.rb b/lib/tkextlib/tile/tscale.rb
index 446344ec..98dc1ab9 100644
--- a/lib/tkextlib/tile/tscale.rb
+++ b/lib/tkextlib/tile/tscale.rb
@@ -3,7 +3,7 @@
 #  tscale & tprogress widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tscrollbar.rb b/lib/tkextlib/tile/tscrollbar.rb
index 621ca48e..9377fd67 100644
--- a/lib/tkextlib/tile/tscrollbar.rb
+++ b/lib/tkextlib/tile/tscrollbar.rb
@@ -3,7 +3,7 @@
 #  tscrollbar widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tseparator.rb b/lib/tkextlib/tile/tseparator.rb
index d4a90e56..4e738a5b 100644
--- a/lib/tkextlib/tile/tseparator.rb
+++ b/lib/tkextlib/tile/tseparator.rb
@@ -3,7 +3,7 @@
 #  tseparator widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tspinbox.rb b/lib/tkextlib/tile/tspinbox.rb
index 47d822e6..4c1077e2 100644
--- a/lib/tkextlib/tile/tspinbox.rb
+++ b/lib/tkextlib/tile/tspinbox.rb
@@ -3,7 +3,7 @@
 #  ttk::spinbox widget  (Tcl/Tk 8.6b1 or later)
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tile/tsquare.rb b/lib/tkextlib/tile/tsquare.rb
index 74129667..c157e6e8 100644
--- a/lib/tkextlib/tile/tsquare.rb
+++ b/lib/tkextlib/tile/tsquare.rb
@@ -3,7 +3,7 @@
 #  tsquare widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tkextlib/tile.rb'
 
 module Tk
diff --git a/lib/tkextlib/tkDND.rb b/lib/tkextlib/tkDND.rb
index c76c48d1..0e7071ac 100644
--- a/lib/tkextlib/tkDND.rb
+++ b/lib/tkextlib/tkDND.rb
@@ -3,7 +3,7 @@
 #  TkDND (Tk Drag & Drop Extension) support
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkDND/shape.rb b/lib/tkextlib/tkDND/shape.rb
index 06d8d64a..78a525e0 100644
--- a/lib/tkextlib/tkDND/shape.rb
+++ b/lib/tkextlib/tkDND/shape.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkDND/tkdnd.rb b/lib/tkextlib/tkDND/tkdnd.rb
index ef195add..a2505ebb 100644
--- a/lib/tkextlib/tkDND/tkdnd.rb
+++ b/lib/tkextlib/tkDND/tkdnd.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkHTML/htmlwidget.rb b/lib/tkextlib/tkHTML/htmlwidget.rb
index c92c224d..da7ef9a3 100644
--- a/lib/tkextlib/tkHTML/htmlwidget.rb
+++ b/lib/tkextlib/tkHTML/htmlwidget.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg.rb b/lib/tkextlib/tkimg.rb
index de4b7212..c3a6e511 100644
--- a/lib/tkextlib/tkimg.rb
+++ b/lib/tkextlib/tkimg.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/bmp.rb b/lib/tkextlib/tkimg/bmp.rb
index 48a3b7ef..d97944da 100644
--- a/lib/tkextlib/tkimg/bmp.rb
+++ b/lib/tkextlib/tkimg/bmp.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'bmp'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/dted.rb b/lib/tkextlib/tkimg/dted.rb
index 30c595c1..e85db196 100644
--- a/lib/tkextlib/tkimg/dted.rb
+++ b/lib/tkextlib/tkimg/dted.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'DTED'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/gif.rb b/lib/tkextlib/tkimg/gif.rb
index ce5f3e94..0712a813 100644
--- a/lib/tkextlib/tkimg/gif.rb
+++ b/lib/tkextlib/tkimg/gif.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'gif'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/ico.rb b/lib/tkextlib/tkimg/ico.rb
index 80656005..91043425 100644
--- a/lib/tkextlib/tkimg/ico.rb
+++ b/lib/tkextlib/tkimg/ico.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'ico'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/jpeg.rb b/lib/tkextlib/tkimg/jpeg.rb
index 24e11a84..5c0cd424 100644
--- a/lib/tkextlib/tkimg/jpeg.rb
+++ b/lib/tkextlib/tkimg/jpeg.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'jpeg'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/pcx.rb b/lib/tkextlib/tkimg/pcx.rb
index 942d5851..7851aa3a 100644
--- a/lib/tkextlib/tkimg/pcx.rb
+++ b/lib/tkextlib/tkimg/pcx.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'pcx'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)#
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/pixmap.rb b/lib/tkextlib/tkimg/pixmap.rb
index b90468a1..88fab8eb 100644
--- a/lib/tkextlib/tkimg/pixmap.rb
+++ b/lib/tkextlib/tkimg/pixmap.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'pixmap'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/png.rb b/lib/tkextlib/tkimg/png.rb
index 76f73297..b9a615b5 100644
--- a/lib/tkextlib/tkimg/png.rb
+++ b/lib/tkextlib/tkimg/png.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'png'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/ppm.rb b/lib/tkextlib/tkimg/ppm.rb
index a6b705e6..26cc402a 100644
--- a/lib/tkextlib/tkimg/ppm.rb
+++ b/lib/tkextlib/tkimg/ppm.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'ppm'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/ps.rb b/lib/tkextlib/tkimg/ps.rb
index 2a450ed1..06e5c334 100644
--- a/lib/tkextlib/tkimg/ps.rb
+++ b/lib/tkextlib/tkimg/ps.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'ps'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/raw.rb b/lib/tkextlib/tkimg/raw.rb
index 2eddbbc4..414e0f46 100644
--- a/lib/tkextlib/tkimg/raw.rb
+++ b/lib/tkextlib/tkimg/raw.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'Raw Data'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/sgi.rb b/lib/tkextlib/tkimg/sgi.rb
index 73a5bce0..d2f5ed3d 100644
--- a/lib/tkextlib/tkimg/sgi.rb
+++ b/lib/tkextlib/tkimg/sgi.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'sgi'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/sun.rb b/lib/tkextlib/tkimg/sun.rb
index bd1ac0e5..49aeb541 100644
--- a/lib/tkextlib/tkimg/sun.rb
+++ b/lib/tkextlib/tkimg/sun.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'sun'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/tga.rb b/lib/tkextlib/tkimg/tga.rb
index 1dd499b9..5ad404ba 100644
--- a/lib/tkextlib/tkimg/tga.rb
+++ b/lib/tkextlib/tkimg/tga.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'tga'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/tiff.rb b/lib/tkextlib/tkimg/tiff.rb
index a2d1d8fc..7dd0a1a6 100644
--- a/lib/tkextlib/tkimg/tiff.rb
+++ b/lib/tkextlib/tkimg/tiff.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'tiff'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/window.rb b/lib/tkextlib/tkimg/window.rb
index 346f246a..1d8ac1d6 100644
--- a/lib/tkextlib/tkimg/window.rb
+++ b/lib/tkextlib/tkimg/window.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'window'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/xbm.rb b/lib/tkextlib/tkimg/xbm.rb
index 0a4deffd..7b0e8a3e 100644
--- a/lib/tkextlib/tkimg/xbm.rb
+++ b/lib/tkextlib/tkimg/xbm.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'xbm'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tkimg/xpm.rb b/lib/tkextlib/tkimg/xpm.rb
index 7708b9d4..a56d3a27 100644
--- a/lib/tkextlib/tkimg/xpm.rb
+++ b/lib/tkextlib/tkimg/xpm.rb
@@ -3,7 +3,7 @@
 #  TkImg - format 'xpm'
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tktable.rb b/lib/tkextlib/tktable.rb
index a957294d..5b039fe4 100644
--- a/lib/tkextlib/tktable.rb
+++ b/lib/tkextlib/tktable.rb
@@ -3,7 +3,7 @@
 #  TkTable support
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tktable/tktable.rb b/lib/tkextlib/tktable/tktable.rb
index 553f4b6c..f196c75c 100644
--- a/lib/tkextlib/tktable/tktable.rb
+++ b/lib/tkextlib/tktable/tktable.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/validation'
 
 # call setup script for general 'tkextlib' libraries
diff --git a/lib/tkextlib/tktrans.rb b/lib/tkextlib/tktrans.rb
index 031eeb9f..74d75187 100644
--- a/lib/tkextlib/tktrans.rb
+++ b/lib/tkextlib/tktrans.rb
@@ -3,7 +3,7 @@
 #  TkTrans support (win32 only)
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/tktrans/tktrans.rb b/lib/tkextlib/tktrans/tktrans.rb
index e6e0e3a7..3b91fe3c 100644
--- a/lib/tkextlib/tktrans/tktrans.rb
+++ b/lib/tkextlib/tktrans/tktrans.rb
@@ -3,7 +3,7 @@
 #  TkTrans support (win32 only)
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/treectrl/tktreectrl.rb b/lib/tkextlib/treectrl/tktreectrl.rb
index d03601b5..0e7c3b6f 100644
--- a/lib/tkextlib/treectrl/tktreectrl.rb
+++ b/lib/tkextlib/treectrl/tktreectrl.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/trofs/trofs.rb b/lib/tkextlib/trofs/trofs.rb
index dc55ec26..ed37acc3 100644
--- a/lib/tkextlib/trofs/trofs.rb
+++ b/lib/tkextlib/trofs/trofs.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/vu.rb b/lib/tkextlib/vu.rb
index 145a0b56..87b91c8d 100644
--- a/lib/tkextlib/vu.rb
+++ b/lib/tkextlib/vu.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/vu/bargraph.rb b/lib/tkextlib/vu/bargraph.rb
index e1d58e95..4a73840a 100644
--- a/lib/tkextlib/vu/bargraph.rb
+++ b/lib/tkextlib/vu/bargraph.rb
@@ -3,7 +3,7 @@
 #  ::vu::bargraph widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # create module/class
 module Tk
diff --git a/lib/tkextlib/vu/charts.rb b/lib/tkextlib/vu/charts.rb
index 66d5cfd3..289c7c20 100644
--- a/lib/tkextlib/vu/charts.rb
+++ b/lib/tkextlib/vu/charts.rb
@@ -3,7 +3,7 @@
 #  charts -- Create and manipulate canvas Add-On Items
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 require 'tk/canvas'
 
 # call setup script  --  <libdir>/tkextlib/vu.rb
diff --git a/lib/tkextlib/vu/dial.rb b/lib/tkextlib/vu/dial.rb
index 7eb5650d..2bd12442 100644
--- a/lib/tkextlib/vu/dial.rb
+++ b/lib/tkextlib/vu/dial.rb
@@ -3,7 +3,7 @@
 #  ::vu::dial widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # create module/class
 module Tk
diff --git a/lib/tkextlib/vu/pie.rb b/lib/tkextlib/vu/pie.rb
index f7e9cf54..7f3b7248 100644
--- a/lib/tkextlib/vu/pie.rb
+++ b/lib/tkextlib/vu/pie.rb
@@ -3,7 +3,7 @@
 #  ::vu::pie widget
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # create module/class
 module Tk
diff --git a/lib/tkextlib/vu/spinbox.rb b/lib/tkextlib/vu/spinbox.rb
index 082ebd1f..37b16f3c 100644
--- a/lib/tkextlib/vu/spinbox.rb
+++ b/lib/tkextlib/vu/spinbox.rb
@@ -6,7 +6,7 @@
 #  a standard spinbox (<= 8.3)
 #  This is the same as the 8.4 core spinbox widget.
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 if (Tk::TK_MAJOR_VERSION < 8 ||
     (Tk::TK_MAJOR_VERSION == 8 && Tk::TK_MINOR_VERSION < 4))
diff --git a/lib/tkextlib/winico.rb b/lib/tkextlib/winico.rb
index 996fc409..e908d466 100644
--- a/lib/tkextlib/winico.rb
+++ b/lib/tkextlib/winico.rb
@@ -3,7 +3,7 @@
 #  winico -- Windows Icon extension support
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'
diff --git a/lib/tkextlib/winico/winico.rb b/lib/tkextlib/winico/winico.rb
index dda0357a..62eccc7c 100644
--- a/lib/tkextlib/winico/winico.rb
+++ b/lib/tkextlib/winico/winico.rb
@@ -4,7 +4,7 @@
 #                               by Hidetoshi NAGAI (nagai@ai.kyutech.ac.jp)
 #
 
-require 'tk'
+require 'tk' unless defined?(Tk)
 
 # call setup script for general 'tkextlib' libraries
 require 'tkextlib/setup.rb'