A thermostat device has a thermo element assembly (10) configured by a piston (3), a thermal expansion unit that changes volume with a change in temperature, to reciprocate a cylinder container (4) with respect to the piston, and a heating element (H) that is provided in a casing of the piston and heats the thermal expansion unit (W) when supplied with current. An assembling hole (11) is provided in the device housing to hold an outer end of the thermo element assembly by causing the outer end to pass therethrough toward the outside of the device. A piston guide (13) is provided at a section on the upper end side of the piston configuring the thermo element assembly. An outer end of the piston guide is integrally joined to the housing side while being inserted from the inside of the device housing (2) into the assembling hole to pass therethrough.