PURPOSE: To make it possible to finely control a valve which is disposed in an intake-air passage to deflect the stream of intake-air, in accordance with the operating condition of an engine, by constituting an actuator for driving the valve with a step motor.

CONSTITUTION: A butterfly valve 17 disposed in the vicinity of an intake-air valve 15 within an intake-air passage 13, has a support shaft 19 piecing through the center of the intake-air passage 13 and is journalled to the wall of the passage 13. In this arrangement a step motor 20 is used as an actuator for driving the support shaft 19. A control unit 21 processes data indicating the operation condition of the engine, such as, for example, the opening degree of a throttle valve in the intake-air passage 13, the negative pressure of intake- air, the rotational speed of the engine, the temperature of the same, and delivers a drive signal to the motor 20. For example, when the amount of intake-air is small, or when the rotational speed of the engine is low and the negative pressure of intake-air is higher, the stroke value of the step motor 20 is reduced to decrease the opening degree of the valve 17, and therefore, the atomization of fuel may be satisfied to intensify the generation of vortices.