PURPOSE: To reduce the number of wiring to a doorphone unit and to terminate the wiring between equipments by driving a disconnection detecting relay by receiving a disconnection detecting signal by the operation of a test button provided at a doorphone slave unit, and short-circuiting both ends of a terminating resistor.

CONSTITUTION: In a housing information panel 1, a continuity signal transmis sion line l3 is connected to a source voltage Vcc via a resistor R, and also, the potential of the continuity signal transmission line l3 is detected by transistors Q2 and Q1, thereby, the control of the disconnection detecting relay Ry 1 is performed. In case of supplying a power ordinarily, a continuously display lamp D is lit and displayed from the source voltage Vcc via the resistor and the continuity signal transmission line l3. Also, in such a state, the transis tor Q2 is turned ON, and the transistor Q1 is turned OFF, and the disconnection detecting relay Ry 1 is turned OFF, and a contact ry 1 is opened. When the test button SW is operated, the both ends of the continuity display lamp D are short-circuited, then, the continuity display lamp D lights out, and the transis tor Q2 is turned OFF, and the transistor Q1 is turned ON, then, the short-circuit detecting relay Ry 1 is turned ON, and the contact ry 1 is closed.