PURPOSE: To prevent the destruction of load due to a voltage abnormality by converting the power supply state from an input power supply into a voltage depending on the result of monitor of a voltage monitoring means and turning on/off the output of a voltage conversion means and the output of a voltage non-conversion means supplied to a same load simultaneously.

CONSTITUTION: When a distribution fuse 3 is fused or dropped off from the position, the output voltage of power output terminals 24, 25 is gradually decreased due to a filter capacitor 22. When the output voltage reaches the lower limit setting voltage of a voltage monitoring circuit 21, the voltage monitoring circuit 21 turns off an electronic switch 16. Thus, the switching power supply 19 is stopped and the latchup of an IC is prevented. Moreover, even when the voltage of a station power supply 1 is boosted and exceeds an upper limit setting voltage of a voltage monitoring circuit 21, the switching power supply 19 is similarly stopped and the output of the power filter section is stopped by the relay contact 20-a of a relay 20. Thus, the destructions of a subscriber circuit 26 and a power supply device 27 are prevented.