diff --git a/History.txt b/History.txt
index e5efe0f268..ca6285acae 100644
--- a/History.txt
+++ b/History.txt
@@ -3,6 +3,9 @@
 * Minor Enhancements
   * Support rb_singleton_class().  Reported by Jeremy Evans.
 
+* Bug Fixes
+  * Tread non-ASCII RDoc files as text.  Bug #28391 by Kouhei Sutou.
+
 === 2.5.9 / 2010-07-06
 
 * Bug Fixes
diff --git a/lib/rdoc/parser.rb b/lib/rdoc/parser.rb
index 9cfda127d9..212b35f341 100644
--- a/lib/rdoc/parser.rb
+++ b/lib/rdoc/parser.rb
@@ -62,12 +62,23 @@ def self.alias_extension(old_ext, new_ext)
     true
   end
 
+  def self.set_encoding(string)
+    if defined? Encoding then
+      if /coding[=:]\s*([^\s;]+)/i =~ string[%r"\A(?:#!.*\n)?.*\n"]
+        if enc = ::Encoding.find($1)
+          string.force_encoding(enc)
+        end
+      end
+    end
+  end
+
   ##
   # Determines if the file is a "binary" file which basically means it has
   # content that an RDoc parser shouldn't try to consume.
 
   def self.binary?(file)
     s = File.read(file, 1024) or return false
+    set_encoding(s)
 
     if s[0, 2] == Marshal.dump('')[0, 2] then
       true
diff --git a/lib/rdoc/rdoc.rb b/lib/rdoc/rdoc.rb
index 9c39354c6c..e712196de3 100644
--- a/lib/rdoc/rdoc.rb
+++ b/lib/rdoc/rdoc.rb
@@ -405,15 +405,7 @@ def document(argv)
 
   def read_file_contents(filename)
     content = open filename, "rb" do |f| f.read end
-
-    if defined? Encoding then
-      if /coding[=:]\s*([^\s;]+)/i =~ content[%r"\A(?:#!.*\n)?.*\n"]
-        if enc = ::Encoding.find($1)
-          content.force_encoding(enc)
-        end
-      end
-    end
-
+    RDoc::Parser.set_encoding(content)
     content
   rescue Errno::EISDIR, Errno::ENOENT
     nil
diff --git a/test/test.ja.rdoc b/test/test.ja.rdoc
index 96e1db93d3..cd01cab37a 100644
--- a/test/test.ja.rdoc
+++ b/test/test.ja.rdoc
@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+
 こんにちは！
 
 初めまして。アーロンと申します。
diff --git a/test/test_rdoc_parser.rb b/test/test_rdoc_parser.rb
index ebb520032d..b97df9e542 100644
--- a/test/test_rdoc_parser.rb
+++ b/test/test_rdoc_parser.rb
@@ -42,6 +42,18 @@ def test_class_binary_eh_marshal
     File.unlink marshal
   end
 
+  def test_class_binary_japanese_text
+    file_name = File.expand_path '../test.ja.txt', __FILE__
+    assert @RP.binary?(file_name)
+  end
+
+  def test_class_binary_japanese_rdoc
+    skip "Encoding not implemented" unless defined? ::Encoding
+
+    file_name = File.expand_path '../test.ja.rdoc', __FILE__
+    assert !@RP.binary?(file_name)
+  end
+
   def test_class_can_parse
     assert_equal @RP.can_parse(__FILE__), @RP::Ruby