A DC/DC converter is driven when an inter-terminal voltage of a low-voltage battery is lower than a determination voltage and an engine is started when the inter-terminal voltage becomes lower than a determination voltage during the driving of the DC/DC converter so that an alternator is driven at a predetermined driving point at which the alternator can he efficiently driven and the DC/DC converter is controlled such that the inter-terminal voltage of the low-voltage battery becomes a rated voltage. Energy efficiency can be improved since the alternator is efficiently driven.