To detect catalyst degradation. A catalyst degradation detecting unit arranged in the exhaust passage of an internal combustion engine, wherein a maximum oxygen storing state where exhaust gas flowing out to the downstream side of a catalyst is in an oxygen-excessive state and a minimum oxygen storing state where it is in an oxygen-deficient state are detected based on an oxygen sensor output. During an oxygen releasing period from the detection of a maximum oxygen storing state until the detection of a minimum oxygen storing state, the target air/fuel ratio of the internal combustion engine is controlled to a rich target air/fuel ratio, and, during an oxygen storing period from the detection of a minimum oxygen storing state until the detection of a maximum oxygen storing state, the target air/fuel ration of the internal combustion engine is controlled to a lean target air/fuel ratio. An oxygen amount released from the catalyst during the oxygen releasing period or an oxygen amount stored into the catalyst during the oxygen storing period is detected as an oxygen storing amount, and catalyst degradation is judged according to the oxygen storing amount. In addition, this unit setsan oxygen storing amount detection condition for correcting a deviation occurring due to the difference in output detecting condition by an oxygen sensor during the oxygen releasing period or the oxygen storing period when catalyst degradation detection is controlled.