A method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine includes comparing the value of a signal indicative of the concentration of an exhaust gas ingredient with a predetermined reference value, detecting from the comparison result a first change in the air-fuel ratio of the mixture supplied to the engine from a value richer than a predetermined value to a value leaner than same, or a second change in the air-fuel ratio of the mixture from a value leaner than the predetermined value to a value richer than same, correcting the value of an air-fuel ratio control signal by increasing or decreasing same by a first predetermined correction value in response to the first change or the second change thus detected, and controlling the air-fuel ratio of the mixture in a feedback manner responsive to the value of the air-fuel ratio control signal thus corrected. A second predetermined correction value, in lieu of the first predetermined correction value, is applied to correction of the air-fuel ratio control signal in response to a selected one of the first change and the second change with a cycle a predetermined number of times as large as the fluctuation cycle of the exhaust gas ingredient concentration-indicative signal.