A thermostat device is configured for installation at a branch point between a main passage and a bypass passage includes a thermostat, a differential pressure valve, and a coupling structure that mechanically connects the thermostat and the differential pressure valve. The thermostat includes a valve seat member, a valve body movable relative to the valve seat member, and a thermo-element having a temperature-responsive material that actuates the valve body to control flow through the main passage. The differential pressure valve includes a valve seat member and a valve body movable to control flow through the bypass passage. The coupling structure includes a holder supporting the differential pressure valve, a flange projecting from the holder, and a biasing member that maintains engagement between the flange and a frame of the thermostat while permitting dimensional tolerance between the coupled components.