To provide a technology of a door unlocking/locking system using a mobile phone with key, which remotely unlocks/locks an operation knob of a door locking device in each facility by means of the mobile phone with key, smartphone with key, or a tablet multi-function terminal unit, to lock/unlock a door.

On a ceiling part in a vehicle, a receiving unit 11 is installed for receiving a transmission signal from a mobile phone B with key. On the basis of the received remote signal, a control part 13 is actuated to switch a remote switch 12 and switch 10. The mobile phone B with key is formed of a key 3 and a body section. The body section includes an electronic circuit, to store/hold or reproduce key information of a specified vehicle A. The switch 10 and the remote switch 12 of the vehicle A can be remotely controlled by operating a push button. Every time the push button is pushed, a locking or unlocking signal is transmitted alternately.