To surely prevent learning errors by determining whether the air-fuel ratio is rich or lean to measure the half-cycle time which shows the duration of the rich or lean state, and permitting renewing of an air-fuel ratio learned value only if the half-cycle time is within a range set by upper and lower limit values.

During engine operation, an ECU 40 calculates a basic fuel injection pulse width from engine speed and the amount of intake air, and sets a variety of increase correcting factors related to correction of cooling water temperature, correction of acceleration and deceleration, etc., based on cooling water temperature, throttle opening, idle output, etc. An air-fuel ratio feedback correcting factor is read from a predetermined address in a RAM 43. Then, based on the engine speed and the basic fuel injection pulse width, an air-fuel ratio learned value is searched by consulting an air-fuel ratio learned value table, and an air-fuel ratio learning correcting factor is set through interpolation. In this case, the air-fuel ratio learned value is renewed only if the half- cycle time which shows the duration of the rich or lean air-fuel ratio is within a predetermined range.