PURPOSE: To reduce a cost of a storage battery by always supplying AC power from a commercial power source to a load, charging the battery, boosting DC power from the battery and supplying it to the load at the time of power interruption.

CONSTITUTION: When a commercial power source 1 is received, a switch 8 is closed to supply AC power to a load 4 through a first rectifier 2-1, an insulating type inverter 3, a second rectifier 2-2, a noninsulating type inverter 9 or a direct line 10, and a storage battery 6 is charged by a charger 7. At the time of power interruption, a relay contact 11 and a thyristor 12 are turned ON thereby to boost DC power from the battery 6 by the inverter 3 and to continue power supply to the load 4 through the rectifier 2-2 and the inverter 9. Thus, the number of cells of the battery 6 can be reduced, and a cost of the battery 6 can be decreased.