diff --git a/ChangeLog b/ChangeLog
index 3c6b4afd7d..67b7f59091 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed Sep 16 03:49:19 2015  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* test/ruby/test_thread.rb (TestThread#test_mutex_synchronize):
+	  insert waste loop for invoking preemptive thread context switch.
+	  [Bug #11496]
+
 Tue Sep 15 19:38:55 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* gc.c (rb_objspace_alloc, rb_objspace_free): define always
diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index be8a28a14b..fe8e2384a0 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -97,21 +97,24 @@ class TestThread < Test::Unit::TestCase
   def test_mutex_synchronize
     m = Mutex.new
     r = 0
-    max = 10
-    (1..max).map{
+    num_threads = 10
+    loop=100
+    (1..num_threads).map{
       Thread.new{
-        i=0
-        while i<max*max
-          i+=1
+        loop.times{
           m.synchronize{
-            r += 1
+            tmp = r
+            # empty and waste loop for making thread preemption
+            100.times {
+            }
+            r = tmp + 1
           }
-        end
+        }
       }
     }.each{|e|
       e.join
     }
-    assert_equal(max * max * max, r)
+    assert_equal(num_threads*loop, r)
   end
 
   def test_mutex_synchronize_yields_no_block_params