PURPOSE: To prevent the leak of steam due to foreign matter and achieve energy economization by forming a thin part ranging from the inner edge to the outer edge on the upper surface of an annular plate and forming a gap between the thin part and a valve disc and discharging foreign matter outside.

CONSTITUTION: In a variable pressure chamber 4 formed of the inner and outer ring valve seats 6 and 7 and a cover member 5, a valve disc 9 is arranged. Further, an edged bimetal ring 17 is arranged at the conical inclined surface part 14 which is formed on the outer periphery of the outer ring valve seat 7. Further, between the valve disc 9 and the bimetal ring 17, an annular plate 18 is arranged. Further, the valve disc 9 is separated from the inner and outer ring valve seats 6 and 7 through the annular plate 18 at a low temperature by the inclined surface part 14 and the spreading/contraction action of the bimetal ring 17 due to the temperature change, and air binding is eliminated. With this constitution, a thin part 18b ranging from inner edge to the outer edge is formed on the upper surface of the annular plate 18. Accordingly, a gap is formed between the thin part 18b and the valve disc 9, and foreign matter is discharged outside.