A controller estimates a first estimated adsorption amount which is an amount of adsorption of ammonia in the SCR catalyst at the time when the SCR catalyst is assumed to be in a predetermined abnormal state, estimates a second estimated adsorption amount at the time when the SCR catalyst is assumed to be in a predetermined normal state, calculates a first slip development temperature based on the first estimated adsorption amount, and calculates a second slip development temperature based on the second estimated adsorption amount. The controller, when carrying out an abnormality diagnosis based on a concentration of ammonia in an exhaust gas at the downstream side of the SCR catalyst, carries out diagnostic temperature control so as to control the temperature of the SCR catalyst to a temperature which is equal to or more than the first slip development temperature and is less than the second slip development temperature.