Index: test/validations_test.rb
===================================================================
--- test/validations_test.rb	(リビジョン 3499)
+++ test/validations_test.rb	(作業コピー)
@@ -365,6 +365,30 @@
     assert !Topic.create("title" => "monkey", "content" => "abc").valid?
   end
 
+  def kcode_scope(kcode)
+    orig_kcode = $KCODE
+    $KCODE = 'UTF8'
+    begin
+      yield
+    ensure
+      $KCODE = orig_kcode
+    end
+  end
+
+  def test_validates_length_of_utf8
+    kcode_scope('UTF8') do
+      Topic.validates_length_of :title, :minimum => 5
+
+      t = Topic.create("title" => "日本語環境", "content" => "whatever")
+      assert t.valid?
+
+      t.title = "日本語"
+      assert !t.valid?
+      assert t.errors.on(:title)
+      assert_equal "is too short (min is 5 characters)", t.errors["title"]
+    end
+  end
+
   def test_validates_length_of_using_minimum
     Topic.validates_length_of :title, :minimum => 5
 
Index: lib/active_record/validations.rb
===================================================================
--- lib/active_record/validations.rb	(リビジョン 3499)
+++ lib/active_record/validations.rb	(作業コピー)
@@ -459,7 +459,7 @@
           message = (options[:message] || options[message_options[option]]) % option_value
 
           validates_each(attrs, options) do |record, attr, value|
-            record.errors.add(attr, message) unless !value.nil? and value.size.method(validity_checks[option])[option_value]
+            record.errors.add(attr, message) unless !value.nil? and value.split(//).size.method(validity_checks[option])[option_value]
           end
         end
       end