While there occurs a steady operation state where an engine operation state (such as intake air quantity) is maintained substantially constant, a high-response intake air quantity sensor (referred to as the sensor, hereafter) is temporarily powered off and is powered on again when a predetermined time elapses after the power-off. An output rising characteristic of the sensor at the time when the sensor is powered on again is sensed and abnormality diagnosis of the sensor is performed based on the sensing value of the output rising characteristic During a power-off period, the engine operation state is controlled using a sensing value (a stored value) of the sensor immediately preceding the power-off. When an abnormality of the sensor is detected, use of the sensor is prohibited and the intake air quantity is estimated based on intake pipe pressure and the like.