Abstract: In a method of controlling an air-fuel ratio for an internal combustion engine, a control section feedback-controls an electronically controlled carburetor in response to a signal from an O.sub.2 sensor which detects the concentration of O.sub.2 in the exhaust gas. When the O.sub.2 sensor outputs a rich signal for a predetermined time period after the air-fuel ratio is leaned upon deceleration of the engine, the rich air-fuel ratio is instantaneously and forcibly leaned by the control section in order to rapidly return it to a proper value. Thereafter, ordinary feedback control is resumed. With this method, the quantity of harmful exhaust gas components such as CO or the like can be reduced without making the operational performance of the engine deteriorate.