An air-fuel ratio control apparatus in which air-fuel ratio detecting signal represents air-fuel ratio in a combustion chamber and an integrated amount increased or decreased in relation to the air-fuel ratio detecting signal is calculated from the air-fuel ratio detecting signal on the basis of parameters to correct fuel amount supplied to an intake system on the basis of the integrated amount. The frequency of the air-fuel ratio detecting signal when the air-fuel ratio in the combustion chamber reaches a predetermined value is defined as the basic frequency. To compensate for change with the passage of time in the output characteristics of an air-fuel ratio detecting sensor, said parameter value is corrected so that the frequency of the air-fuel ratio detecting signal becomes the basic frequency.