diff --git a/ChangeLog b/ChangeLog
index 707c6e3113..57feff07e5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Feb 10 00:47:07 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* test/ruby/envutil.rb (assert_no_memory_leak): new assertion to
+	  check memory leak by invoking child ruby process and watch its
+	  memory size.
+
 Thu Feb  9 23:41:44 2012  CHIKANAGA Tomoyuki  <nagachika00@gmail.com>
 
 	* test/pathname/test_pathname.rb (test_binread): add assertion to
diff --git a/test/-ext-/string/test_modify_expand.rb b/test/-ext-/string/test_modify_expand.rb
index 5fee7b3580..6a18560bd4 100644
--- a/test/-ext-/string/test_modify_expand.rb
+++ b/test/-ext-/string/test_modify_expand.rb
@@ -4,26 +4,12 @@ require_relative '../../ruby/envutil'
 
 class Test_StringModifyExpand < Test::Unit::TestCase
   def test_modify_expand_memory_leak
-    before = after = nil
-    args = [
-      "--disable=gems", "-r-test-/string/string",
-      "-I"+File.expand_path("../../..", __FILE__),
-      "-rruby/memory_status",
-      "-e", <<-CMD
+    assert_no_memory_leak(["-r-test-/string/string"], <<-PRE, <<-CMD, "rb_str_modify_expand()")
       s=Bug::String.new
-      size=Memory::Status.new.size
-      puts size
+    PRE
+      size = $initial_size
       10.times{s.modify_expand!(size)}
       s.replace("")
-      puts Memory::Status.new.size
     CMD
-    ]
-    status = EnvUtil.invoke_ruby(args, "", true) do |in_p, out_p, err_p, pid|
-      before, after = out_p.readlines.map(&:to_i)
-      Process.wait(pid)
-      $?
-    end
-    assert_equal(true, status.success?)
-    assert_operator after.fdiv(before), :<, 2
   end
 end
diff --git a/test/ruby/envutil.rb b/test/ruby/envutil.rb
index ef5f65c747..69b037b4d8 100644
--- a/test/ruby/envutil.rb
+++ b/test/ruby/envutil.rb
@@ -184,6 +184,28 @@ module Test
         assert(msg === stderr, "warning message #{stderr.inspect} is expected to match #{msg.inspect}")
       end
 
+      def assert_no_memory_leak(args, prepare, code, message=nil, limit: 1.5)
+        token = "\e[7;1m#{$$.to_s}:#{Time.now.strftime('%s.%L')}:#{rand(0x10000).to_s(16)}:\e[m"
+        token_dump = token.dump
+        token_re = Regexp.quote(token)
+        args = [
+          "--disable=gems",
+          "-r", File.expand_path("../memory_status", __FILE__),
+          *args,
+          "-v", "-",
+        ]
+        cmd = [
+          'END {STDERR.puts '"#{token_dump}"'"FINAL=#{Memory::Status.new.size}"}',
+          prepare,
+          'STDERR.puts('"#{token_dump}"'"START=#{$initial_size = Memory::Status.new.size}")',
+          code,
+        ].join("\n")
+        out, err, status = EnvUtil.invoke_ruby(args, cmd, true, true)
+        before = err.sub!(/^#{token_re}START=(\d+)\n/, '') && $1.to_i
+        after = err.sub!(/^#{token_re}FINAL=(\d+)\n/, '') && $1.to_i
+        assert_equal([true, ""], [status.success?, err], message)
+        assert_operator(after.fdiv(before), :<, limit, message)
+      end
     end
   end
 end