A method of controlling an air-fuel ratio of an air-fuel mixture for an internal combustion engine includes the steps of sensing the intake air temperature and the operating state and rotating speed of the engine using various detectors. When the intake air temperature is above a predetermined temperature, when the engine is in the idle operating state, and when its rotating speed is below a predetermined speed, the control circuit ceases a normal control in which the air-fuel ratio of the air-fuel mixture is controlled to be a value in a theoretical air-fuel ratio range, and instead controls the air-fuel ratio to be a predetermined rich air-fuel ratio. With this method, a decrease in the absolute amount of fuel supplied to the engine due to a reduction in intake air quantity in the idle operating state when the intake air temperature is high is avoided, which in turn prevents a reduction in the rotating speed of the engine and ensures stability of idling.