diff --git a/ChangeLog b/ChangeLog
index 7a1f5375ba035a..85043b00dd93e9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Aug 25 03:08:57 2007  Koichi Sasada  <ko1@atdot.net>
+
+	* prelude.rb: fix Mutex#synchronize definition.
+
 Sat Aug 25 02:08:45 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* array.c (rb_ary_s_try_convert): a new class method to convert
diff --git a/prelude.rb b/prelude.rb
index 423c9c423ec403..6451b8f74aef21 100644
--- a/prelude.rb
+++ b/prelude.rb
@@ -2,13 +2,11 @@
 # Mutex
 
 class Mutex
-  class Mutex
-    def synchronize
-      self.lock
-      yield
-    ensure
-      self.unlock
-    end
+  def synchronize
+    self.lock
+    yield
+  ensure
+    self.unlock
   end
 end