An electronic controller integrates a rate of progress dK of thermal degradation of exhaust purifying catalyst at every predetermined time period, to calculate degree of thermal degradation K of the exhaust purifying catalyst, when the state of thermal degradation of the exhaust purifying catalyst is to be detected. By comparing the degree of thermal degradation K with a defect determining value S, whether the exhaust purifying catalyst is defective because of thermal degradation or not is determined. In calculating the degree of thermal degradation K, the electronic controller calculates the rate of progress dK of thermal degradation in the predetermined time period based on the integrated value of rate of progress dK of thermal degradation up to the last time (degree of thermal degradation K) and on the temperature T of exhaust purifying catalyst at that time, and adds the same to the integrated value up to the last time.