Calculation of a present air-fuel ratio correction coefficient correction value &Dgr;FAF (i) is based on a control parameter calculated by an ECU, a change in air-fuel ratio detected by an air-fuel ratio sensor, a deviation of an actual air-fuel ratio from a target air-fuel ratio and an immediately preceding air-fuel ratio correction coefficient correction value &Dgr;FAF (i−1) Then, a present air-fuel ratio correction coefficient FAF (i) is found by adding the present air-fuel ratio correction coefficient correction value &Dgr;FAF (i) to an immediately preceding air-fuel ratio correction coefficient FAF (i−1). As a result, the air-fuel ratio correction coefficients is not be thrown into confusion and no phenomenon of the air-fuel ratio being thrown into confusion occurs even if the control parameter is changed in accordance with operating conditions of the engine.