To improve repeatability and to eliminate the hysteresis of feedback sensor.

A control system must repeatedly establish a position of the same throttle valve during idling, acquire fuel efficiency and prevent speed reduction. A sensor decides the position of the throttle valve or other engine parameters concerning the position of the throttle valve and makes an analog signal. This information is processed by an analog/digital converter arranging the signal at a discrete level. A control device receives the signal, compares it with a desired position of the throttle valve or a point allotted between the two discrete levels showing the engine parameters. Consequently, the signal is not probably equal to the allotted point. As an error does not reach zero, the control device corrects it in accordance with the comparison after repeating all of the processes.