A control apparatus for an internal combustion engine can detect degradation of a three-way catalyst with high accuracy without causing deterioration in an exhaust. A pair of first and second air fuel ratio detectors are disposed in an exhaust system at locations upstream and downstream of the three-way catalyst for detecting a first and a second air fuel ratio of an exhaust gas. A target oxygen change amount calculator calculates a target oxygen change amount of the three-way catalyst, and an oxygen change amount calculator calculates an oxygen change amount of the three-way catalyst from an amount of exhaust gas passing through the three-way catalyst and the first air fuel ratio. An air fuel ratio operator inversely controls the air fuel ratio to a rich side and a lean side with a prescribed air fuel ratio width each time the oxygen change amount reaches a target oxygen change amount.