PURPOSE: To achieve a theoretical air-fuel ratio faster in a transition period of an engine operation by making the cycle of an air-fuel ratio control more frequent on the basis of signals from an O2 sensor and throttle sensor in an exhaust system.

CONSTITUTION: Each signal of an O2 sensor 19 an throttle sensor 20 is transmitted to a controller 21 so as to control solenoid valves 14, 15 which act to vary the air breed of a carburetter. A signal is transmitted from the O2 sensor 19 to a shaping circuit 23 in way of an amplifying circuit 22, into which circuit 23 a slice level is also input. By setting its center valve, an addition of signals is performed in an addition circuit 28 into which a signal corresponding to each acceleration or deceleration state is fed from the throttle sensor 20. At the same time, a change in an air- fuel ratio caused by a change in the acceleration or deceleration degree is anticipated at a super-position circuit 30 and a comparator circuit 31. On the basis of the anticipated valve, the air-fuel ratio during the acceleration or deceleration is controlled.