To provide a thumb-turn guard having a cover on a thumb-turn as a crime prevention measure to prevent the thumb-turn picked up by a thumb and turned from being operated from the outside and a function for locking the thumb-turn itself to prevent it from being turned when locking a key from an inner side of a door.

This thumb-turn guard is composed of a thumb-turn passing-through hole, a base having the cover of opening and closing type capable of covering the thumb-turn inserted into the thumb-turn passing-through hole, a movable lever capable of switching locking the thumb-turn and its releasing when closing the cover, a fixing means fixed on the base when closing the cover, and a fixing means for fixing the base on the door.