PURPOSE: To converge the engine rotation quickly to a target level, by varying the rotary speed of a step motor for controlling the opening of a throttle valve to a low level under a low opening area of throttle valve and to a high level under a high opening area.

CONSTITUTION: An arm 9 secured to the output shaft 8 of a step motor 7 is engaged with a throttle valve lever 4 loaded in the closing direction of a returning coil spring 5 secured to one end of the valve shaft 3 of a throttle valve 2. Said arm 9 is arranged at the opening side of the lever 4 so that the motion of the lever 4 in the opening direction is limited or the lever 4 is pushed in the closing direction. The step motor 7 is controlled by a control unit 10 according to a rotary speed signal and a throttle valve opening signal, where the rotary speed of the step motor 7 is varied according to the opening of throttle valve to a low level in a low opening area of the throttle valve 2 while to a high level in a high opening area of the throttle valve 2.