An alternating current that is to be supplied to a motor generator is controlled by controlling an inverter using a target output of the motor generator and a result of detection of an angle-of-rotation sensor, and a voltage step-up is controlled in accordance with the target output of the motor generator. During operation of the motor generator, a stepped-up voltage obtained through the step-up converter is set to be higher (S13) when learning of the point of origin of the angle-of-rotation sensor has yet to be completed (S11), than when learning of the point of origin of the angle-of-rotation sensor has been completed. This eliminates malfunctions occurring in motor operation when learning of the point of origin of the angle-of-rotation sensor has yet to be completed.