To provide a thermostat device to be mounted on a vehicle (at its engine etc.), embodied in a simple structure using a minimum necessary number of component parts and capable of enhancing the assembling easiness etc.

The device is equipped with a first coolant passage 2B, a second coolant passage 2D, a casing 2 having a thermostat accommodation part where the first and second coolant passages are in communication, a lid 3 having a third coolant passage 2C in communication to the thermostat accommodation part and covering it, and a thermostat 10 having a thermo-element 10h to be advanced and retreat in accordance with the temperature change of a coolant liquid flowing in the thermostat accommodation part. A temperature sensor 20 to sense the coolant temperature is installed confronting the inside of the thermostat accommodation part, and a sensor mounting part 21 fitted internally with this temperature sensor is provided rigidly at the inner end of the lid, and at the outer end, a lead connection part 25 drawn out from the temperature sensor is provided rigidly.