To solve problems of difficult production and repair of duplicate keys and insufficient preparation of duplicate keys due to high functions and high prices thereof under the circumstances that lock breakage of doors of a house, an automobile, etc., has been increased in recent years and keys and locks using new technology have been developed to cope with the breakage, and to provide an auxiliary locking device which is employed in addition to an existing key and an existing lock, and gives feeling of ensured locking and security, taking into consideration the common knowledge that each door has two or three locks.

The auxiliary locking device is mounted on the inside of a door in addition to a main lock mounted on a front side of the door. According to the auxiliary locking device, a lock can be mounted on or dismounted from the door from the outside of the door by using a joint-type chain or an ordinary chain.