diff --git a/package-lock.json b/package-lock.json
index 717235a..c08aafa 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -28,6 +28,7 @@
         "just-extend": "6.2.0",
         "just-safe-set": "4.2.1",
         "mdast-util-to-string": "4.0.0",
+        "outvariant": "1.4.2",
         "patch-package": "8.0.0",
         "prettier": "3.2.4",
         "prettier-plugin-astro": "0.13.0",
@@ -8012,6 +8013,11 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/outvariant": {
+      "version": "1.4.2",
+      "resolved": "https://registry.npmjs.org/outvariant/-/outvariant-1.4.2.tgz",
+      "integrity": "sha512-Ou3dJ6bA/UJ5GVHxah4LnqDwZRwAmWxrG3wtrHrbGnP4RnLCtA64A4F+ae7Y8ww660JaddSoArUR5HjipWSHAQ=="
+    },
     "node_modules/p-limit": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/p-limit/-/p-limit-5.0.0.tgz",
diff --git a/package.json b/package.json
index e6cd8c3..3582164 100644
--- a/package.json
+++ b/package.json
@@ -38,6 +38,7 @@
     "just-extend": "6.2.0",
     "just-safe-set": "4.2.1",
     "mdast-util-to-string": "4.0.0",
+    "outvariant": "1.4.2",
     "patch-package": "8.0.0",
     "prettier": "3.2.4",
     "prettier-plugin-astro": "0.13.0",
diff --git a/src/pages/404.astro b/src/pages/404.astro
index 3ef5a8f..87e5fc5 100644
--- a/src/pages/404.astro
+++ b/src/pages/404.astro
@@ -1,12 +1,12 @@
 ---
+import { invariant } from 'outvariant';
 import PageHeader from '../components/PageHeader.astro';
 import Layout from '../layouts/Layout.astro';
-import invariant from '../lib/tiny-invariant';
 
 const title = 'ページが存在しません';
 
 const searchURL = new URL('https://www.google.com/search');
-invariant(Astro.site);
+invariant(Astro.site, 'Invariant failed');
 searchURL.searchParams.set('q', `site:${Astro.site.host}`);
 ---
 
diff --git a/src/pages/[...slug].astro b/src/pages/[...slug].astro
index 0e48495..66f9b69 100644
--- a/src/pages/[...slug].astro
+++ b/src/pages/[...slug].astro
@@ -1,9 +1,9 @@
 ---
 import { getCollection } from 'astro:content';
+import { invariant } from 'outvariant';
 import PageHeader from '../components/PageHeader.astro';
 import Layout from '../layouts/Layout.astro';
 import { renderDateToHtml } from '../lib/render-date-to-html';
-import invariant from '../lib/tiny-invariant';
 
 export async function getStaticPaths() {
   return (await getCollection('blog')).map((entry) => ({
@@ -20,6 +20,7 @@ const { Content, remarkPluginFrontmatter } = await entry.render();
 invariant(
   typeof remarkPluginFrontmatter.description === 'string' ||
     typeof remarkPluginFrontmatter.description === 'undefined',
+  'Invariant failed',
 );
 ---
 
diff --git a/src/pages/posts-feed.xml.ts b/src/pages/posts-feed.xml.ts
index 9c1cce5..c0995ae 100644
--- a/src/pages/posts-feed.xml.ts
+++ b/src/pages/posts-feed.xml.ts
@@ -1,15 +1,15 @@
 import rss from '@astrojs/rss';
 import type { APIContext } from 'astro';
 import { getCollection } from 'astro:content';
+import { invariant } from 'outvariant';
 import { SITE_TITLE, SITE_DESCRIPTION } from '../consts';
-import invariant from '../lib/tiny-invariant';
 
 export async function GET(context: APIContext) {
   const blogEntries = (await getCollection('blog'))
     .toSorted((a, b) => a.data.pubDate.valueOf() - b.data.pubDate.valueOf())
     .toReversed();
 
-  invariant(context.site);
+  invariant(context.site, 'Invariant failed');
 
   return rss({
     title: SITE_TITLE,
@@ -22,6 +22,7 @@ export async function GET(context: APIContext) {
         invariant(
           typeof remarkPluginFrontmatter.description === 'string' ||
             typeof remarkPluginFrontmatter.description === 'undefined',
+          'Invariant failed',
         );
 
         return {