A temperature of a catalyst disposed in an exhaust system is raised to a target bed temperature by supplying an unburned fuel component to the catalyst. A learned value is updated based upon a catalyst bed temperature under a temperature increase control and the target bed temperature so that the learned value corresponds to a difference between the respective temperatures. Whether the learned value is out of a proper range or not is determined whenever the learned value is updated. Abnormality is determined when determination that the learned value is out of the proper range is made over several successive updates of the learned value.