PURPOSE: To prevent a storage battery from overdischarging by providing a voltage detector, and a power source supplying breaker composed of a switch for turning OFF a power supply path in cooperation with the detector.

CONSTITUTION: When a commercial power source 1 is interrupted, DC power is supplied from a storage battery to an electronic exchange system 9. When the interruption of the power source 1 is prolonged, the discharge of the battery 4 is advanced, and the supplying voltage is reduced. In this case, when the voltage of the battery 4 is reduced lower than a predetermined value, a voltage detector 7 detects that the supplying voltage is reduced lower than the predetermined value thereby to operate a switch 6, thereby breaking a power supply path between the terminal 11 and the terminal 12 of a power source interrupting unit 5. Thus, the system 9 is disconnected from the battery 4, thereby preventing the battery 4 from overdischarging.