To prevent a cylinder from being rotated from the outside by forming a groove in a shaft on an indoor side, forming a hole having a shape of a combination of a circle and a square in a L-shaped plate central portion, and sliding the L-shaped plate along the groove, to thereby prevent the shaft from being rotated.

An L-shaped plate 3 has a hole 4 formed therein at a central portion thereof, which hole has a shape of a combination of a circle and a square. A shaft 2 of a lock has a groove 1 at a base of the shaft in a manner being arranged on an upper side and a lower side of the base when locked. The shaft 2 of the lock is inserted into the hole 4 of the L-shaped plate 3, and conformed to the groove 1 of the shaft 2. When the lock is used, a knob of the lock is rotated from the inside of the door to lock the door normally, and the L-shaped plate 3 is slid to fix the shaft 2 of the lock. Thus, the lock cannot be opened from the outside.