Control diagnostic apparatus for an internal combustion engine

To prevent the deterioration of exhaust emissions of an internal combustion engine due to air-fuel ratio variation among multiple cylinders, and to identify an abnormal cylinder during an abnormality of cylinder air-fuel ratio variation. An control apparatus 108 of an internal combustion engine comprising upstream air-fuel ratio detection means 101 for detecting upstream air-fuel ratio of the catalyst 107 that purifies exhaust emissions discharged from multiple cylinders, and configured to control air-fuel ratio of the multiple cylinders based on the upstream air-fuel ratio, wherein the air-fuel ratio variation among the multiple cylinders is increased and the upstream air-fuel ratio is controlled to become rich. Further, an abnormality of the air-fuel ratio variation among the multiple cylinders and an abnormal cylinder are identified based on an output of an air-fuel ratio sensor 102 in downstream of the catalyst 107 when the air-fuel ratio variation is increased, or an estimated value of an air-fuel ratio (central air-fuel ratio) at which the purification efficiency of the catalyst 107 becomes optimal.