Abstract: A method of effecting feedback control of the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine to a first predetermined value, by correcting a basic fuel supply quantity by the use of a correction coefficient variable in response to the output of an exhaust gas ingredient concentration sensor, when the engine is operating in a predetermined feedback control region. A predetermined engine low-load operating region outside the predetermined feedcback control region is defined by at least one parameter representing load on the engine. When the engine enters the predetermined low-load operating region, the correction coefficient is gradually decreased until the air-fuel ratio of the air-fuel mixture is increased to a second predetermined value leaner than the first predetermined value.