PURPOSE: To cope with the overdischarge of a main battery by charging the main battery and an auxiliary battery having a small capacity by a generator when both the batteries are not overdischarged, and raising the voltage of the auxiliary battery by a converter thereby to charge it when the main battery is overdischarged.

CONSTITUTION: A generator 3 charges a main battery 2 during the operation of an engine, and charges an auxiliary battery 101 having a small capacity through switches 4, 110 and a diode 102. When the battery 2 is overdischarged and the generator 3 is not started at the time of starting the engine, a switch 104 of the battery 101 is continuously pressed. A control circuit 112 outputs to fire an LED 108 thereby to energize a Tr circuit 115, to close a switch 6, to apply the voltage of the battery 101 to a DC/DC converter 107, to raise the voltage, and to apply it to the terminal of the battery 2, thereby starting the engine. Thus, even if the main battery is overdischarged, the engine is immediately started.