On the basis of a correlation between exhaust gas temperature detected by an exhaust gas temperature sensor and exhaust gas temperature estimated according to an engine operating condition, it is detected whether or not abnormality occurs in the exhaust gas temperature sensor. Namely, when a change characteristic of the detected exhaust gas temperature follows a change characteristic of the estimated exhaust gas temperature, it is determined that the exhaust gas temperature sensor is normal. On the other hand, when the change characteristic of the detected exhaust gas temperature does not follow that of the estimated exhaust gas temperature, it is determined that the exhaust gas temperature sensor is abnormal.