A generator has a positive and a negative output terminals across which a battery is connected, and another output terminal for generating an output power earlier than the positive output terminal when the generator begins to generate an output power. A field energizing circuit is connected to the other output terminal and a battery voltage responsive circuit to thereby establish a field excitation when the generator begins to generate the output power at the other output terminal and when the battery voltage is below a predetermined value, whereby the field excitation can be carried out earlier and battery discharge through the field energizing circuit is prevented during the time when the engine is stopped and a key switch remains closed.