PURPOSE: To improve safety by screwing a removable cover to a lock cylinder insertion port being formed at an outer shell from the inside by a wrench, and at the same time by blocking a wrench insertion hole by a cam plate when a lock is set to a closing position.

CONSTITUTION: From an opening part being provided at a front half part 11 of a casing for covering a curved shackle lock 16, a cylinder 21 for activating the shackle lock 16 is inserted, and is screwed by a wrench 28 from the inside with a retention cover 15. Also, at a back half part 12 of the easing, a shackle driving cam 17 is mounted, and at the same time an insertion port 12a of the wrench 28 is blocked by the cam 17 when the shackle lock 16 is set to a locking position. Furthermore, to mutually fix the front and back part members 11 and 12 of the casing for forming in one piece, a lock retainer ring with a lateral part that can be bent is used as a circular base part, and the lateral part is bent for covering and is subjected to spot welding, thus improving the safety of the lock.