When a key cylinder is caused to come off a body, a regulating lever moves. As a result of such movement, a block member moves to engage with an engagement groove of an engagement body, so that the rotation of a cam shaft can be blocked. In addition, a lock piece enters between the block member and a cover. Even if the regulating lever is broken, the rotation of the cam shaft is blocked by the block member, and even if one attempts to move the block member in a disengagement direction, the movement of the block member is blocked by the lock piece.