PROBLEM TO BE SOLVED: To provide a failure diagnosis system for an exhaust temperature sensor that can accurately diagnose the exhaust temperature sensor.SOLUTION: A diagnosis system 21 for diagnosing failure of an exhaust temperature sensor 134 installed in an exhaust passage 125 of an internal combustion engine EG includes: a non-combustion detection section 212 for detecting a non-combustion state where combustion of the internal combustion engine is stopped; a re-combustion detection section 212 for detecting a re-combustion state where the combustion is resumed after the non-combustion state; and diagnosis sections 215, 216 for diagnosing failure of the exhaust temperature sensor. When the non-combustion state is detected, the diagnosis sections prohibit the failure diagnosis of the exhaust temperature sensor, and when the re-combustion state is detected after the non-combustion state, the diagnosis sections allow the failure diagnosis of the exhaust temperature sensor after a lapse of predetermined dela
