diff --git a/ChangeLog b/ChangeLog
index db7e5e77d0..fba82c0db8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue May 17 20:20:49 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* vm_core.h (rb_thread_struct): add volatile to
+	  transition_for_lock because it is not protected by lock.
+
 Tue May 17 20:08:53 2011  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* LEGAL (missing/{elf,tgamma,lgamma_r}.c): they've been replaced by
diff --git a/vm_core.h b/vm_core.h
index c5417d9381..e302e62a4b 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -435,7 +435,7 @@ typedef struct rb_thread_struct {
     struct rb_unblock_callback unblock;
     VALUE locking_mutex;
     struct rb_mutex_struct *keeping_mutexes;
-    int transition_for_lock;
+    volatile int transition_for_lock;
 
     struct rb_vm_tag *tag;
     struct rb_vm_protect_tag *protect_tag;