PURPOSE: To feed voltage stably by performing constant control of output voltage according to the output from means for amplifying the difference between the output voltage from a power supply and a reference voltage, and lowering the reference voltage when input voltage to the power supply drops below the lower control limit.

CONSTITUTION: Voltage of a commercial power supply 1 is subjected to A/D conversion 2 in order to charge a battery 3 and further subjected to D/A conversion and voltage transformation 6 thus producing an AC voltage through an AC filter 7. A voltage reference circuit 21 produces a reference voltage based on a DC voltage detected through a DC voltage detecting circuit 22 and the reference voltage is provided to an error amplifier 13. Difference between the output voltage from an output voltage detecting circuit 11 and the reference voltage is then amplified 13 in order to control the modulation width of a PWM modulation circuit 15 and to control the inverter 5 through a gate amplifying circuit 16 thus producing a constant voltage. When power interruption endures for a long term and the battery voltage drops to the lower control limit, reference voltage to be outputted from the voltage reference circuit 21 is lowered and voltage control operation is continued.