PURPOSE: To provide a system capable of controlling the opening of throttle valve by the turning force of a step motor greater than the biasing force of a return coil spring in response to increase in the depression angle of accelerator pedal, with high sensitivity, even if the accelerator pedal is depressed rapidly.

CONSTITUTION: Rotor R of step motor M is connected coaxially to turning shaft 22 of throttle valve 20. Here, arrangement is such that throttle valve 20 can be opened stepwise in normal direction against the force of return sping 23 when exciting windings C1WC4, C1, C2, C3 and C4 of the step motor M are excited two at a time successively in this order. An electric control circuit for the step motor M is constituted by constant-voltage circuit 30, accelerator sensor 40, A-D converter 50, computor 60 and driving circuit 70. The computor 60 is designed to determine the turning direction of motor M from the difference between the depression angle of accelerator pedal 10 and the turning angle of motor M at the time immediately before depression of accelerator pedal 10 and to calculate the set value representing the time width corresponding to the turning angle of motor M.