To avoid a foreign matter from being retained between an outer ring valve seat and a valve disc.

A conical inclined part 12 is formed at the external periphery of the outer ring valve seat 7, and an upper-side flat part 15 and a lower-side flat part 16 are formed at the upper end and the lower end of the inclined part 12, respectively, a bimetal ring 13 having an end is arranged, and an interposing ring 14 having an end is arranged between the bimetal ring 13 and the valve disk 11. At a low temperature, the bimetal ring 13 is contraction-closed and upwardly moves the inclined part 12, lifts the valve disk 11 via the interposing ring 14 and seat-separates and valve-opens it from the inner and outer valve seats 6, 7, and places it on the upper-side flat part 15. The foreign matter between the outer ring valve seta 7 and the valve disk 11 is made to flow out to the outside of the outer ring valve seat 7 through a gap between portions having ends of the interposing ring 14, and thus prevented from being retained in the upper part of the outer ring valve seat 7.