To provide a thumb-turn locking device, which is attached simply to an existing thumb-turn lock without labor by a post-attachment as it is and has a simple structure and the cost of which is reduced.

A locking device body A is assembled detachably near a thumb-turn knob 20 for a door D. A movable arm 11 moved between a locking place, where the thumb-turn knob is made to collide directly and a rotation is controlled when the tumb-turn knob for the thumb-turn lock S is turned, and a locking-releasing place, where the rotation is permitted, and an arm driving means 12 such as a driving motor M driving the movable arm to the locking place or the locking releasing place in response to a locking requirement signal when the requirement signal or a locking-releasing requirement signal is received are mounted on the locking device body. The thumb-turn locking device is constituted so that the locking requirement signal or the locking releasing requirement signal is transmitted to the driving motor by a remote control by using a portable signal transmission means and the movable arm is driven automatically.