A catcher is supported on a base to be capable of reciprocal movement between a standby position where a striker can be caught or released and a final position, and moves slidingly toward the final position after rotating at a fixed angle from the standby position during forward movement from the standby position. Holding of the state in which the catcher is in the standby position, and the aforementioned sliding movement, are accomplished by urging of an urging device provided on the base. The aforementioned holding is released and forward rotation of the catcher is started when the striker is caught by the catcher in the standby position by forward movement of a movable body, and this forward movement is assisted by the aforementioned urging from mid-course of this forward movement.