A fault diagnostic apparatus enables to improve estimate accuracy of temperature of an object to be examined and to improve fault diagnosis accuracy of a temperature detector for detecting a temperature of the object to be examined. The fault diagnostic apparatus for diagnosing a fault of a water temperature sensor for detecting a temperature of cooling water comprises a fault diagnostic device for diagnosing a fault of the water temperature sensor based on a change in temperature detected by the water temperature sensor in a situation where a parameter to be a chief source of heating the cooling water (such as an engine temperature) decreases.