The thermostat installing structure is provided, according to which a thermostat can be installed stably in the circulation path of cooling medium, and the circulation path of cooling medium can be also fastened to the body of an internal combustion engine without causing any damage to the function of the thermostat. The thermostat (5) comprises a frame (12) formed integrally with an annular supporting substrate (6) having the inner circumferential edge portion serving as a valve seat (6a) and regulating a projecting head portion (7a) of an extendable rod (7), whereby the thermostat is installed between the circulation path of cooling medium and the cooling medium channel of the internal combustion engine in a manner that the circulation path of cooling medium and the internal combustion engine body are integrally fastened by fixing the thermostat at a predetermined position in the circulation path of cooling medium with the fixing means and thereafter fitting a gasket.