A method of controlling in a feedback manner the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine having a first fuel injection valve and a second fuel injection valve each operating in a different operating region, during operation of the engine in an air-fuel ratio feedback control region, by the use of a coefficient which varies with the output of an exhaust gas ingredient concentration sensor. In a first and a second operating regions within the air-fuel ratio feedback control region in which the first and second fuel injection valves operate, respectively, a first average value and a second average value of values of said coefficient obtained in the respective regions are calculated, and stored, respectively. When the engine has shifted to the first or and second operating regions, a value based on the corresponding first or second average value is used as an initial value of the coefficient to therely start the feedback control. Preferably, in a third operating region defined as a predetermined period of time which elapses after the engine has shifted from the second operating region to the first operating region, a third average value of values of the coefficient obtained in the third operating region is calculated and stored, and when the engine shifted to the third operating region, the third average value is used as an initial value of the coefficient to thereby start the feedback control.