PURPOSE: To simplify structure by fixing a key mounting part for mounting a cylinder key by a rotatably mounted lock arm.

CONSTITUTION: Two bending parts 12a formed on the lower end of a key mounting part 12 of a rectangular plate material is locked by a receiving box 11. A shaft 14 is protruded on the inner surface of the receiving box 11 on the key mounting part 12 upper part, and a locking part 12b is formed nearly in the center of the upper end part. A bending part 13a is formed on one end part side of a lock arm 13, and rotatably installed to the shaft 14. At the time of final shipment, the lock arm 13 is energized by a twisted coil spring 15, and its top end is engagingly inserted to the locking part 12b of the key mounting part 12, and this key mounting part 12 is exchangeably installed by the lock arm 13. In field, the bending part 13a of the lock arm 13 is rotated to release the top end of the lock arm 13 from the locking part 12b, and a cylinder key is installed.