To normally operate communication equipment for security services, even if a power supply line and a communication line are connected to either of terminals of the equipment.

A first switching means 6 and a second switching means 7 are provided at the first and the second terminals 4, 5 to connect the power supply line A or the communication line B. The switching means 6, 7 switchably connect these terminals portions to either of a system power supply 2 or a signal processor 3. A voltage detector 8 discriminates whether the power supply line A is connected to the second terminal 5. Then a switching control unit 9 switchably controls the first and the second switching means 6, 7 so that the power supply line A may be connected to the system power supply 2, and the communication line B may be connected to the signal processor 3.