1. An air-fuel ratio control apparatus for an internal combustion engine, characterized by comprising: estimation means for estimating, 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 the engine, an estimation value which is an emission of at least one specific component contained in exhaust gas flowing out of the exhaust purifying catalyst unit or at least one representative value indicating the state of the exhaust gas flowing out of the exhaust purifying catalyst unit; target setting means for setting a target state regarding the estimating value estimated by the estimation means; and air-fuel ratio control means for controlling the air-fuel ratio of the engine in such a manner that the estimation value estimated by the estimation means reaches the target state set by the target setting means.