To largely reduce a part cost by eliminating a DC-DC converter, to make voltages of two systems employed to a vehicle keep optimum voltage values independently, and to properly charge a battery even if an inverter device is in a failure .

The field control circuit 63 of the inverter device 11 comprises a changeover switch 66 that can switch voltage systems of a 42V system and a 14V system. A motor generator control device 33 for controlling the inverter device 11 monitors an inverter control unit 61, forcibly confines a function of the inverter device 11 to only a rectifying function that varies an AC from the motor generator 15 to a DC of a prescribed voltage when the inverter control device 61 has an abnormality in operation, and also performs the idle-up of an engine 1. The field control circuit 63 monitors the inverter control unit 61, too, and forcibly confines the function of the inverter device 11 to only the rectifying function in a similar way when the inverter control unit 61 has an abnormality in operation.