To provide a locking method capable of simply increasing the effect of the prevention of crimes to the existing door at a low cost without being replaced with a new door or being processed.

An auxiliary lock is mounted to the door by means of a locking metal fitting 1 and a lock on the market to make it more difficult to illegally open the lock. Concretely, the locking metal fitting 1 is mounted to an indoor side fixed metal fitting, and the lock on the market is locked in a hole 2 in the other end in a state to close the door to stop the opening of the door.