To provide lifetime by preventing deterioration of a battery by supplying the battery with electricity at a constant voltage and charging the battery.

In an engine power generator automatically supplying batteries 3 with electricity from the engine power generator 1 and charging the batteries, constant-voltage control is conducted at the initial of charging, and constant- current is conducted at the end of charging. Constant-current control is conducted for a set time, a set voltage is obtained by charging the number of revolutions of an engine when conducting constant-voltage control, and the number of revolutions of the engine is kept constant in the case of constant- current control.