To provide a device capable of easily checking the state of locking without directly turning a door knob.

A check device a having a contact point e is installed in the receiving seat f of the dead bolt g of a lock used widely at present. In the locked state, the dead bolt g is inserted into the receiving seat f to separate the contact point e. Accordingly, the alarm device does not operate. When the lock is released and the dead bolt g is separated from the receiving seat f, the contact point e is brought into contact. Then, the alarm device b operates, and notifies of the unlocked state by sounding a buzzer or flashing a light.