An ECU executes a cylinder-by-cylinder air-fuel-ratio control in which an air-fuel-ratio of each cylinder is estimated based on a detection value of an air-fuel-ratio sensor to adjust the air-fuel-ratio of each cylinder. Further, the ECU computes a learning value of a correction quantity for each cylinder, which is obtained by executing the cylinder-by-cylinder air-fuel-ratio control. Then, the ECU determines whether the estimated air-fuel-ratio has converged according to whether the estimated air-fuel-ratio of each cylinder has been closer to a target value than a specified value for not less than a specified time period. A computation of the learning value is prohibited until the estimated air-fuel-ratio has converged. Therefore, it can be avoided to compute the learning value based on the fuel correction quantity that is obtained when the estimated air-fuel-ratio has not converged yet.