Whether or not an electrically heated catalyst is normal is detected by accurately detecting whether the temperature of the electrically heated catalyst has gone up. To this end, provision is made for an air fuel ratio control device that adjusts an air fuel ratio of an exhaust gas flowing into the electrically heated catalyst to a rich air fuel ratio, at the time of starting of an internal combustion engine, a downstream side detection device that is arranged at the downstream side of the electrically heated catalyst and detects a concentration of oxygen in the exhaust gas, and a determination device that determines whether the electrically heated catalyst is electrically energized, based on the time at which the oxygen concentration detected by the downstream side detection device changes to a value indicating a rich air fuel ratio at the time when the air fuel ratio of the exhaust gas is adjusted by the air fuel ratio control device to the rich air fuel ratio, after starting of the internal combustion engine.