PURPOSE: To improve the controllability by obtaining the control quantity of a control signal to a driving device for driving the actuator to control the opening degree of a throttle valve based on the function of the predetermined operating quantity in the driving device and he detected operating quantity.

CONSTITUTION: The opening degree of a throttle valve 1 is controlled by an actuator 2 via a negative pressure in an air intake pipe, and said negative pressure is controlled by a driving device 3 including a solenoid valve 8 performing an ON-OFF duty actuation. The solenoid valve 8 is controlled by a control device 5. In this case, it is judged by an engine load state whether the throttle valve 1 is to be furthermore opened than that in feedback or not. And, in case of the judgement of YES, a ratio of IS/IN is computed, wherein IN is a current value applied to a valve 8 detected by a actuation state detecting device 4 and IS is the predetermined current value. Then, a compensated duty DG is obtained by the ratio of IS/IN, and added to an output duty together with a throttle opening degree increasing component ΔD.