diff --git a/ChangeLog b/ChangeLog
index f8306bfd1b..4aa364a275 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Nov 20 10:12:46 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* thread.c (rb_threadptr_async_errinfo_active_p): added a small
+	  comment.
+
 Tue Nov 20 10:08:45 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* thread.c, vm_core.h: big rename th to cur_th when works only
diff --git a/thread.c b/thread.c
index d21fc1a36d..38251afd56 100644
--- a/thread.c
+++ b/thread.c
@@ -1476,12 +1476,20 @@ rb_threadptr_async_errinfo_deque(rb_thread_t *th, enum interrupt_timing timing)
 int
 rb_threadptr_async_errinfo_active_p(rb_thread_t *th)
 {
-    if (th->async_errinfo_queue_checked || rb_threadptr_async_errinfo_empty_p(th)) {
+    /*
+     * For optimization, we don't check async errinfo queue
+     * if it nor a thread interrupt mask were not changed
+     * since last check.
+     */
+    if (th->async_errinfo_queue_checked) {
 	return 0;
     }
-    else {
-	return 1;
+
+    if (rb_threadptr_async_errinfo_empty_p(th)) {
+	return 0;
     }
+
+    return 1;
 }
 
 static VALUE