A diagnostic apparatus for an internal combustion engine. An exhaust purification section includes an exhaust purifier disposed in an exhaust passage of the internal combustion engine, for purifying an exhaust emission, and a purifier activation section configured to control engine operating parameters of the internal combustion engine to promote activation of the exhaust purifier during the internal combustion engine being cold. The diagnostic apparatus is configured to determine a state of the exhaust purifier, to determine a total purifier out emission quantity as a quantity of the exhaust emission exiting the exhaust purifier during the internal combustion engine being cold, in accordance with the state of the exhaust purifier, and to determine normality of the exhaust purification section in accordance with the total purifier out emission quantity.