diff --git a/README.md b/README.md
index fa61be6..9bd9cbd 100644
--- a/README.md
+++ b/README.md
@@ -24,7 +24,7 @@ See also
 # Requirements
 
 - MKL-DNN Library (0.14 or later)
-- ProtocolBuffers
+- ProtocolBuffers (3.5.1 is checked)
 
 # Build