An incorrect or wrong diagnosis in abnormality diagnosis of a sensor is suppressed. Provided at the downstream side of an NOx selective reduction catalyst is a sensor which detects NOx and ammonia in an exhaust gas flowing out of the NOx selective reduction catalyst, and in which the NOx and the ammonia react with each other. An amount of decrease in an output of the sensor due to a reaction of the NOx and the ammonia in the sensor is calculated based on the NOx and the ammonia flowing out of the NOx catalyst, and an abnormality diagnosis of the sensor is carried out based on this amount of decrease in the output of the sensor.