PURPOSE: To prevent the generation of impact at the time of changeover by providing a hydraulic chamber, disposed at the periphery of an orifice valve, for providing an action delaying characteristic and a needle valve for closing an orifice.

CONSTITUTION: An orifice valve 14 with an orifice 13 is movably enclosed in a main passage 7 by means of a spring 11 and a spring 12, a heat sensitive deformable member. In the orifice valve 14, the orifice 13 is closed by a needle valve 17 when the delivery pressure or the temperature reaches the specified value. A hydraulic chamber 35A for providing an action delaying characteristic is provided at the periphery of the orifice valve 14. Accordingly, impact is not generated at the time of changeover, nor is frequent changeover action generated.