An air-fuel ratio control apparatus estimates, on the basis of an exhaust air-fuel ratio of exhaust gas flowing into an exhaust purifying catalyst unit disposed in an exhaust passage of an engine, an emission of at least one specific component contained in exhaust gas flowing out of the exhaust purifying catalyst unit. The air-fuel ratio control apparatus performs the estimation by use of a model, and controls the air-fuel ratio in such a manner that the estimation value reaches a target state. The model is previously determined in consideration of the mass balance of the specific component.