A control apparatus such as an ECU, that controls the generator of a vehicle, measures the engine rotation speed and generator rotation speed while the engine is idling, and calculates and stores the ratio of the speed values. Thereafter when the vehicle is being driven and the current value of generator rotation speed is required, it is calculated based on the stored ratio value and the currently measured speed of the engine. The processing load on the ECU is thereby reduced, since generator rotation speed measurement processing is performed only in the idling condition.