To prevent loss of synchronism of a synchronous motor controlling a flow rate control valve and obtain excellent emission performance and comfortable operation performance.

Changed amount of driving speed of a synchronous motor for every output timing is calculated by a driving speed changed amount arithmetic means 48. When changed amount of driving speed exceeds prescribed threshold, driving speed of the motor is limited by extending a period of prescribed output timing for outputting a driving signal or changing the excitation phase of a step motor and further continuing the state for a prescribed period, by a driving speed relaxation means 45.