To improve the safety of a lock by disabling picking.

An engaging projection 13 is formed on a thumb-turn shaft 5 which is guided into a thumb-turn guide cylinder 2 in a manner being roratable and movable in a longitudinal direction and is biased in a direction approaching a cylinder lock 12, and an engaging depression 4 for engaging with the engaging projection 13 to inhibit movement of the projection 13 in the circumferential direction is formed in the thumb-turn guide cylinder 2. On the other hand, one end of an operating pin 17 is internally inserted into a key hole internal end of a cylinder lock 12 in a manner being movable in the longitudinal direction, and the other end of the operating pin 17 is abutted on the internal end of the thumb-turn shaft 5. At the time of locking, the engaging projection 13 is engaged with the engaging depression 4, to thereby prevent the thumb-turn shaft 5 from being rotated. On the other hand, at the time of unlocking, the operating pin 17 is extruded by the front end of a duplicate key 19 in a direction of the thumb-turn shaft 5, whereby the engaging projection 13 is pulled out of the engaging depression 4.