diff --git a/ChangeLog b/ChangeLog
index 7d673bbdf7..398e0e6366 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Oct 24 12:15:02 2013  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* thread.c (rb_thread_terminate_all): add a comment why infinite
+	  sleep is safe.
+
 Thu Oct 24 07:41:42 2013  Aman Gupta <ruby@tmm1.net>
 
 	* gc.c: add new initial_growth_max tuning parameter.
diff --git a/thread.c b/thread.c
index f33e630afb..757fca203b 100644
--- a/thread.c
+++ b/thread.c
@@ -440,6 +440,10 @@ rb_thread_terminate_all(void)
 
 	TH_PUSH_TAG(th);
 	if ((state = TH_EXEC_TAG()) == 0) {
+	    /*
+	     * Thread exiting routine in thread_start_func_2 notify
+	     * me when the last sub-thread exit.
+	     */
 	    native_sleep(th, 0);
 	    RUBY_VM_CHECK_INTS_BLOCKING(th);
 	}