diff --git a/ChangeLog b/ChangeLog
index affcd131c7..c6eb2275f8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Feb 27 22:09:24 2009  Tanaka Akira  <akr@fsij.org>
+
+	* lib/test/unit/assertions.rb: don't define assert_not_* if already
+	  overridden.
+
 Fri Feb 27 22:02:34 2009  Tanaka Akira  <akr@fsij.org>
 
 	* util.c: fix SEGV by test_time.rb with
diff --git a/lib/test/unit/assertions.rb b/lib/test/unit/assertions.rb
index de357d0fea..35348d87be 100644
--- a/lib/test/unit/assertions.rb
+++ b/lib/test/unit/assertions.rb
@@ -122,8 +122,10 @@ EOT
         super if !caller[0].rindex(MiniTest::MINI_DIR, 0) || !obj.respond_to?(meth)
       end
 
-      instance_methods(true).grep(/\Arefute_/) do |m|
-        alias_method(('assert_not_' << m.to_s[/.*?_(.*)/, 1]), m)
+      ms = instance_methods(true).map {|sym| sym.to_s }
+      ms.grep(/\Arefute_/) do |m|
+        mname = ('assert_not_' << m.to_s[/.*?_(.*)/, 1])
+        alias_method(mname, m) unless ms.include? mname
       end
 
       def build_message(head, template=nil, *arguments)