PURPOSE: To prevent any interruption of a control state monitoring during control over an electric lock from occurring by transmitting an alternating current signal, varying its frequency at each type of control, to a two-wire signal line where an electric lock control DC signal is always impressed, and discriminating the frequency.

CONSTITUTION: A control unit 1 and a lock operating terminal unit 2 are connected by a two-wire signal line 4, and a direct current signal driving an electric lock 3 is always impressed. Next, when a locking operation switch and an unlocking operation switch both are closed of each circuit, the unit 1 outputs respective operation signals different in frequency each to the signal line 4, and the unit 2 outputs an operation signal to an electric lock driver as well. In succession, when a closed door detection switch, a locking detection switch and unlocking detection switch are all closed, this unit 2 outputs respective monitor signals different in frequency each to the signal line 4. In addition, the unit 1 detects these signals, outputting an operation signal to a control state monitor. With this constitution, electric lock control and control state monitoring are simultaneously performable in this way, so that system cost is sharply reducible.