1. A method for controlling an air-fuel ratio of a gas mixture supplied to an engine by regulating a basic fuel amount so as to conform said air-fuel ratio to a target air-fuel ratio, comprising: determining the basic fuel amount in response to the operating state of the engine; measuring the air-fuel ratio in exhaust gas of the engine with a sensor; calculating an actual fuel amount which will cause the air-fuel ratio in the exhaust gas to conform to the target air-fuel ratio, comprising: calculating a first feedback correction value in accordance with the air-fuel ratio measured by said sensor, and correcting the basic fuel amount by the feedback correction value, calculating a learning value in accordance with the feedback correction value, and correcting the basic fuel amount by the learning value, and initializing the feedback correction value after calculation of a learning value which is different than a prior learning value, wherein the feedback correction value is initialized to a second value which equals the first feedback correction value plus the difference between the prior learning value and the calculated learning value; and mixing the actual fuel amount with air to form the gas mixture.