In an air-fuel ratio feedback control device for a fuel injection control system, a catalyst deterioration diagnosing means for carrying out deterioration diagnosis of the catalyst is equipped with a diagnosis permission judging means for judging permission of the deterioration diagnosis of the catalyst on the basis of a driving area of the internal combustion engine, an air-fuel ratio varying means for varying the target upstream air-fuel ratio at the catalyst deterioration diagnosis time, a catalyst deterioration judging means for judging the deterioration of the catalyst on the basis of the downstream air-fuel ratio detection output at the catalyst deterioration diagnosis time, and a catalyst deterioration diagnosis prohibiting means for setting a deterioration diagnosis prohibition time of the catalyst after the catalyst deterioration diagnosis.