To provide a door locking/unlocking method which can eliminate the trouble of carrying a special key for locking and unlocking a door by enabling the door to be locked and unlocked by remote control, or allows a home security system or the like to permit, from a distant place, the entrance into a house or the like.

An automatic lock 4 which performs the action of locking or unlocking a door 5 in response to a lock/unlock signal is provided. In accordance with the operation of a cellular phone 1 connected with a main telephone 2 via a public telephone line 6, the lock/unlock signal is transmitted to the automatic lock 4 via the public telephone line 6 to lock or unlock the door 5. Alternatively, a receiving unit for receiving electric waves from the cellular phone 1 is provided, and in accordance with the operation of the cellular phone 1 the lock/unlock signal is transmitted to the automatic lock 4 via the receiving unit to lock or unlock the door 5.