An oxygen amount (an oxygen storage amount OSA) stored by metal cerium during a storage cycle is obtained basis on an output value of an A/F sensor 18 in a time period immediately before an exhaust air-fuel ratio (a downstream side A/F) detected by the A/F sensor 18 shifts to a lean region, and degradation relating to OSC of the three-way catalyst is detected. In the storage cycle, a target air-fuel ratio is set so that an air-fuel ratio of exhaust emission flowing into an S/C 14 changes from rich to lean. Therefore, the downstream side A/F shifts from stoichiometry to the lean region at a time t 1 after reaching stoichiometry. When the three-way catalyst degrades, the downstream side A/F shifts to the lean region from stoichiometry at a time t 4 . Therefore, the calculated oxygen storage amount OSA becomes extremely smaller as compared with that at a normal time.