CATALYST DETERIORATION DETECTION SYSTEM

The catalyst deterioration detection system comprises an air-fuel ratio detection device detecting an air-fuel ratio of an exhaust gas flowing out from the catalyst , an air-fuel ratio control part , and a deterioration judgment part . The air-fuel ratio control part is configured to perform a lean control making the air-fuel ratio of the inflowing exhaust gas leaner than a stoichiometric air-fuel ratio and a rich control making the air-fuel ratio of the inflowing exhaust gas richer than the stoichiometric air-fuel ratio. The deterioration judgment part is configured to calculate an amplitude of an air-fuel ratio of an exhaust gas flowing out from the catalyst due to the lean control and the rich control based on an output of the air-fuel ratio detection device and judge that the catalyst is deteriorating if the amplitude is equal to or greater than a threshold value.