PURPOSE: To prevent an engine stall, by setting the desired opening of an intake- air adjusting valve to a higher value than that at a stationary state as moving to an idle, when keeping that desired opening so as to have the valve adjusted to desired idle revolving speed by means of feed-back control.

CONSTITUTION: A deflection c between output of an engine speed sensor 300, which detects speed a of an engine 100, and desired engine speed b is inputted to an integrator 400 and, when an idle switch 700 detects a state of idling, outputs a throttle valve desired position signal e. In addition, when a positioning deflection f between the said position signal e and the output of a throttle position sensor 500, which detects an actual position d of a throttle valve 101 of the engine 100, is outputted to a driver 600, idle revolving speed feedback control takes place. A compensator 900 receiving the output of a speed regulation switch 800 adds a certain value to integrator output e and attenuates and adding value in time of deceleration with time elapsing; the engine stall is thus obviated.