diff --git a/content/treestyletab/treestyletab.js b/content/treestyletab/treestyletab.js
index 74515f4e..99113de8 100644
--- a/content/treestyletab/treestyletab.js
+++ b/content/treestyletab/treestyletab.js
@@ -633,6 +633,8 @@ try{
 		window['piro.sakura.ne.jp'].autoScroll.processAutoScroll(aEvent);
 
 		var info = this.getDropAction(aEvent, session);
+		if (!(info.position & this.kDROP_ON))
+			return;
 
 		var setEffectAllowedFunc;
 		var observer = aTabBrowser;