To provide an auxiliary lock for a crescent lock, which enables the existing crescent lock to be easily locked.

A lock pin 3 of the auxiliary lock 2 is inserted from between a latching fitting 5 and a mounting base 6 below the crescent lock 4, and a locked state is brought about by using the auxiliary lock 1. The latching fitting 5 is sandwiched from both sides between the auxiliary locks 1 and 2 and the lock pin 3, and the auxiliary locks 1 and 2 are fixed because the lock pin 3 is hooked by the crescent lock 4. Since the whole of the crescent lock is protected by the auxiliary locks 1 and 2, the operation of the crescent lock is disabled.