Abstract: An on-off duty ratio of a transistor that increases and decreases a current applied to a field coil of an alternator is monitored. When the duty ratio is 100%, the electric generation capability of the alternator is at a maximum, and the battery current of a battery exceeds a predetermined current limit. Under these conditions, a torque limit is set to tighten a limit on the torque output from a motor. The motor is thus controlled within a range determined by the set torque limit. As a result, the motor can be controlled while the electric power outputs and requirements of the alternator, the battery and the motor are balanced, without having to directly detect the high voltage electric power of the alternator. Accordingly, the flow of excessive current in the battery can be impeded, and deterioration of the battery can be avoided.