In a four-wheel drive vehicle, front wheels 14L and 14R are driven by an engine 1, and rear wheels 15L and 15R are driven by an electric motor 5. A high-power alternator 2 is driven by the engine 1, and electric power generated from the alternator 2 drives the motor 5. In addition to controlling the power generation of the high-power alternator 2 and the driving of the motor 5, a 4WD CU 100 estimates an induced voltage E of the motor 5 from a voltage MHV of the motor and from an output current Ia of the high-power alternator, and estimates a rotating speed Nm of the motor from estimation results on the induced voltage E.