Abstract: An air-fuel ratio of each cylinder is estimated on the basis of the detection value of an air-fuel ratio sensor disposed in an exhaust confluent portion. An air-fuel ratio correction quantity is computed on the basis of the estimated air-fuel ratio of each cylinder and the air-fuel ratio correction quantity is learned by smoothing processing. The air-fuel ratio is corrected on the basis of the cylinder air-fuel ratio correction quantity, whereby the air-fuel ratio is controlled for each cylinder so as to reduce a variation in the air-fuel ratio between the cylinders. When a state in which the correction quantity of any cylinder is outside a specified range continues for a while during a period in which the air-fuel ratio is controlled for each cylinder, the correction quantity is prohibited from being learned, whereby the cylinder air-fuel ratio correction quantity is prevented from being erroneously learned.