PURPOSE: To improve the degradation judging accuracy of a catalyst and each exhaust sensor, and reduce the dispersion quantity of catalyst degradation instrumentation at the materialized time of a catalyst degradation judging condition and also reduce the generation of harmful components in exhaust gas at the change-over time of the second feedback control value between the unmaterialized time and materialized time of the catalyst degradation judging condition, and moreover reduce the dispersion of the catalyst degradation judgment value.

CONSTITUTION: A catalyst degradation judging device is provided with a setting control means. At the materialized time of a catalyst degradation judging condition, the second feedback control value at the time of degradation judgment is set in correspondence with the second feedback control value at the unmaterialized time of the catalyst degradation judging condition, that is, at the normal time The sum of rich reversal delay time and lean reversal delay time is set to the fixed value, and the rate between the rich reversal delay time and lean reversal delay time is set to the fixed value. The correction value of the first feedback control value is set larger than that at the unmaterialized time of the catalyst degradation judging condition, and the integral judging time of the second feedback control value at the materialized time of the catalyst degradation judging condition is set shorter than that at the unmaterialized time of the catalyst degradation judging condition.