PURPOSE: To mount a secondary power supply with a small capacity by inhibiting forcibly the operation of a subscriber circuit when the voltage of the secondary power supply is lower than the preset value during power failure based on the priority of the subscriber circuit so as to minimize the power consumption by the subscriber circuit.

CONSTITUTION: A battery 2 is charged by a charger 1 during the reception of AC power. In this case, a DC/DC converter 3 is activated by the output of the charger 1 and supplies the DC output for various operation control to the subscriber circuit. A comparator 4 generates an output V0 at '0' level in response to the output of the charger 1. If power failutre takes place, the output of the charger is missing and the power supply from the battery 2 to the DC/DC converter 3 and the comparator 4 is started instead. When the voltage of the battery 2 is dropped to, e.g., 53V as times elapses, the output V0 of the comparator 4 is inverted to '1' level. The voltage V0 is given to a relay 7 through a strap terminal 5 and a buffer 6 to close contacts k1-1 and k1-2 of the relay 7 and the telephone set 8 is disconnected forcibly from the subscriber circuit.