To decrease the number of parts such as an input member and an operation obstructing element of the knob side.

In a crime prevention thumb-turn, a lock groove for regulating the rotation of a knob member is formed on the surface of a fixed cylinder having shaft holes for the knob member and the thumb-turn shaft extending from the surface to the back, and the knob member is built in the lock groove, an insert hole formed in the thumb-turn shaft and the large shaft hole to move in the axial direction. When the thumb-turn shaft is rotated by the power from the lock box side, the knob member is pushed outward through a frictional engagement of the thumb-turn shaft and the knob member and against the spring force of an energizing spring provided in the insert hole of the thumb-turn shaft, and the knob part is released from the lock groove.