A thermostat failure detection device sequentially calculates a normal-time minimum water temperature on an assumption that a thermostat is normal and an internal combustion engine is operated in a state where the engine water temperature is less likely to rise, sequentially calculates a failure-time maximum water temperature on an assumption that the thermostat is in a stuck-open failure state and the internal combustion engine is operated in a state where the engine water temperature is likely to rise, determines the failure of the thermostat if the engine water temperature is lower than the normal-time minimum water temperature, determines the normality of the thermostat if the engine water temperature is higher than the failure-time maximum water temperature, and determines neither the normality nor the failure if the engine water temperature is between the normal-time minimum water temperature and the failure-time maximum water temperature.