PURPOSE: To prevent resonance and unstable actuation in a servo system from occurring by determining a final feedback controlled variable on the basis of output from two decision means to determine the feedback controlled variable by both proportional and integral items.

CONSTITUTION: A target opening signal of a throttle valve 1 out of a throttle controller 4, an actuator position signal out of a first position sensor 17 and a throttle valve opening signal out of a second position sensor 18 are all inputted into a central processing unit 9 which compares each output value of these position sensors 17, 18 with the target opening, and determines a feedback controlled variable by proportional control, derivative control and integral control on the basis of the deviation. Then, a final feedback controlled variable is determined in the shape of totaling these controlled variables, whereby a current value to be made flowing in a motor 3 and the current direction are calculated. With this constitution, resonance and unstable actuation in a servo system is prevented from occurring, and that accuracy and responsiveness in feedback can be secured.