THERMOSTAT FAILURE JUDGMENT DEVICE

A failure judgment device for a thermostat 16 arranged in a circulation passage 13 of cooling water 12 in an engine 1 includes a water-temperature sensor 18 for detection of a temperature of cooling water 12 having passed through the engine 1, and a controller 19 which monitors rise in temperature of the cooling water 12 at cold startup by a detection signal 18a inputted from the water-temperature sensor 18 and which calculates a predicted water temperature in the engine 1 as from beginning of the startup to judge failure of the thermostat 16 when the predicted water temperature reaches a valve opening temperature of the thermostat 16 with the temperature of the cooling water 12 being not beyond a threshold value.