DIAGNOSING APPARATUS AND DIAGNOSING METHOD FOR INTERNAL COMBUSTION ENGINE

An electronic control unit is used for an internal combustion engine that executes a specific-cylinder fuel cutoff process in which combustion in part of cylinders is stopped and combustion is continued in the remaining cylinders. The electronic control unit diagnoses whether there is a misfire anomaly in the internal combustion engine based on a misfire frequency of the internal combustion engine. The electronic control unit executes a normality restoration diagnosing process of diagnosing, in a state in which it is diagnosed that there is a misfire anomaly, that the misfire anomaly has been resolved when a measured value of the misfire frequency becomes less than or equal to a specified normality restoration determination value. The normality restoration diagnosing process is configured to not diagnose, during the execution of the specific-cylinder fuel cutoff process, that the misfire anomaly has been eliminated.