A power generator has a battery and an alternator driven by an engine. The battery assists a power energy of the alternator. An inverter circuit is connected to an output of a rectifying circuit. A DC/DC converter boosts a voltage of the battery and inputs the boosted voltage into a constant power regulator. The constant power regulator boosts an input voltage and secures a certain power. An output voltage of the rectifying circuit is monitored by a monitoring means and an output voltage of the battery is monitored by a monitoring means. When the output voltage of the rectifying circuit is equal to or less than a rated voltage, an auxiliary power value corresponding to the remaining amount of the battery is set as a power target value of the constant power regulator.