PURPOSE: To stabilize engine speed in time of idling by always compensating a full-close reference position at an idling speed controller used with a single throttle valve.

CONSTITUTION: A neutral signal (XNSW), an air conditioner signal (XAC), an electric load signal (WELS), an accelerator position signal and each of engine speed (NE) and water temperature (THW) signals are inputted into a central processing unit 23 of an electronic control unit 20 via an input circuit 22. In this central processing unit 23, throttle owing serving as a target engine speed in an idling time when the actual engine speed is prestored is calculated, and it is compared with both upper and lower limit values of the throttle valve in time of the idling prestored, and on the basis of the result, a full-close reference position of the throttle valve is compensated. With this constitution, this full-close reference position in the throttle valve of a throttle valve 10 comes to be always compensated in consequence.