To improve fuel saving effects, and to prevent overdischarge of a battery device for vehicle driving, in a hybrid car in which an auxiliary battery is charged by an battery device for vehicle driving to be charged by an external power source prior to an alternator.

When the amounts of electricity accumulated in a battery device 10 for vehicle driving is a predetermined value or more, a switching part 16 is held at the battery device 10 side for vehicle driving, and an auxiliary battery 7 is charged with the electricity accumulation energy of the battery device 10 for vehicle driving. In this case, an alternator 3 is prevented from imposing any load on the battery device 10 for vehicle driving by disconnecting the alternator. Also, when the amounts of electricity accumulated in the battery device 10 for vehicle driving is decreased to predetermined quantity or less, the switching part 16 is switched to the alternator 3 side, and the charging means of the auxiliary battery 7 is switched from the battery device 10 for vehicle driving to the alternator 3 so that the amounts of electricity accumulated in the battery device 10 for vehicle driving can be prevented from being decreased any more.