diff --git a/ChangeLog b/ChangeLog
index 82cff7e61f..3e7aa70c6f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed May 19 23:19:30 2010  Shugo Maeda  <shugo@ruby-lang.org>
+
+	* lib/net/imap.rb (disconnect): closes the socket of a Net::IMAP
+	  object only when it is not closed.  [ruby-dev:41350]
+
 Wed May 19 20:09:38 2010  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* vm_eval.c (rb_f_caller): return [] instead of nil when the function
diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 0f6a4372d5..3404700297 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -311,7 +311,11 @@ module Net
         @receiver_thread.raise(e)
       end
       @receiver_thread.join
-      @sock.close
+      synchronize do
+        unless @sock.closed?
+          @sock.close 
+        end
+      end
       raise e if e
     end