PURPOSE: To prevent a lock from being left unlocked by informing a carrier of a locking/unlocking key by the sound of alarm that he has left the key unlocked when he has departed a certain distance from the lock, for example, even in the case that he departs temporarily from a locking place, leaving a lock unlocked, or he goes out.

CONSTITUTION: Electromagnetic waves with a predetermined wave length 4a are transmitted from a transmitting device 4 provided in a locking device 2 simultaneously with the unlocking of the locking device 2, whereupon a portable alarming device 3 receives the waves 4a. When the alarming device 3 is separated a predetermined distance or more from the locking device, an alarm is emitted to the outside by the alarming means installed in the device 3.