PURPOSE: To inhibit the fluctuation of output voltage at the time of changeover by suddenly changing the output control signal of a voltage controller only by a section corresponding to a level, where battery voltage immediately before the changeover of battery operation and the voltage of a DC filter capacitor are compared, and controlling output voltage.

CONSTITUTION: In an uninterruptible power supply, the voltage of a DC filter capacitor 18 and the voltage of a battery 16 are compared. When a switch 17 is conducted by an electric signal corresponding to the voltage comparison, the output signal of an output voltage detector 21 is increased to a value corresponding to the ratio of the voltage of the battery 16 to the voltage of the DC filter capacitor 18, and an output corresponding to the output signal of a voltage controller 22 is acquired previously by a voltage controller 29. Accordingly, the control delay of the voltage controller 22 can be removed transiently, thus inhibiting the fluctuation of the output voltage of output terminal voltage 15 even when the voltage of the DC filter capacitor 18 is changed suddenly.