An oil temperature sensor is externally mounted on a side plate of an oil pan cover that constitutes an external cover for an oil pan apparatus. The oil temperature sensor can output a signal in accordance with the oil temperature prevailing near a thermostat valve apparatus in a second chamber. If the second chamber oil temperature rises immediately after startup of an engine, a CPU judges that an open valve failure has occurred, that is, the thermostat valve apparatus is stuck open. If the oil temperature of the second chamber is not raised when an adequate amount of time has elapsed after the end of a warm-up operation, the CPU judges that a closed valve failure has occurred, that is, the thermostat valve apparatus is stuck closed.