PURPOSE: To aim at miniaturizing a control unit and reducing its cost, by designing an input signal to an air-fuel ratio control apparatus to be taken from only a binary signal generator composed of an on/off switch in accordance with O2 sensor and engine conditions.

CONSTITUTION: A control unit 13 consisting of a filter 131, a microprocessor 132 and a driver 133 varies the duty ratio of a solenoid 9 and controls an air-fuel ratio so as to make it constant. The filter 131 is inputted with only a binary signal from an O2 sensor 3 which outputs low voltage or high voltage according to the air-fuel ratio on whether rich or lean, a vacuum switch 121 being turned on or off on whether below or above the prescribed value, an idle switch 122 and a water temperature switch 123. Therefore, an A/D converter is no longer necessary and, what is more, microprocessor miniaturization into a small bit can be promoted in this way.