diff --git a/data/bitclust/catalog/ja_JP.UTF-8 b/data/bitclust/catalog/ja_JP.UTF-8
index 961df2f..3b5c683 100644
--- a/data/bitclust/catalog/ja_JP.UTF-8
+++ b/data/bitclust/catalog/ja_JP.UTF-8
@@ -114,3 +114,7 @@ module %s
 %sモジュール
 object %s
 %sオブジェクト
+This reference manual is for a version of Ruby that is no longer maintained.
+このマニュアルは既にメンテナンスが終了したバージョンの Ruby を対象としています。
+Latest version
+最新版URL
diff --git a/data/bitclust/template/layout b/data/bitclust/template/layout
index d133166..43ae0e3 100644
--- a/data/bitclust/template/layout
+++ b/data/bitclust/template/layout
@@ -13,6 +13,12 @@
   <link rel="search" type="application/opensearchdescription+xml" title="<%= _('Ruby %s Reference Manual', ruby_version()) %>" href="<%=h opensearchdescription_url() %>">
 </head>
 <body>
+<% if eol?() %>
+<p class="eol-warning">
+<%= _('This reference manual is for a version of Ruby that is no longer maintained.') %>
+<a href="https://www.ruby-lang.org/ja/documentation/"><%= _('Latest version') %></a>
+</p>
+<% end %>
 <%= yield %>
 <div id="footer">
   <a rel="license" href="https://creativecommons.org/licenses/by/3.0/">
diff --git a/lib/bitclust/screen.rb b/lib/bitclust/screen.rb
index 7216d89..0e2ffaa 100644
--- a/lib/bitclust/screen.rb
+++ b/lib/bitclust/screen.rb
@@ -314,6 +314,12 @@ def meta_description
       %Q(<meta name="description" content="">)
     end
 
+    MINIMUM_SUPPORTED_RUBY_VERSION = '3.1'
+
+    def eol?
+      /\A\d(\.\d)+\z/ =~ ruby_version && ruby_version < MINIMUM_SUPPORTED_RUBY_VERSION
+    end
+
     private
 
     def default_encoding
diff --git a/theme/default/style.css b/theme/default/style.css
index a9e563a..8ed7fae 100644
--- a/theme/default/style.css
+++ b/theme/default/style.css
@@ -406,6 +406,17 @@ hr {
  right: 10px;
 }
 
+.eol-warning {
+    background-color: #aa3333;
+    color: white;
+    padding: 0.4em;
+}
+
+.eol-warning a {
+    color: white;
+    text-decoration: underline;
+}
+
 @media only screen and (max-width:425px) {
     table.entries tr {
         display: block;
