To allow a reverse bimetal to quickly reverse to be convex downward at high temperature.

A disklike reverse bimetal 12 with a larger diameter than that of a valve disk 11 and a smaller diameter than that of an inner peripheral wall of a variable pressure chamber 4 is mounted on an upper surface of the valve disk 11 and coupled on a center part of the valve disk 11. A bimetal seat 14 comprising a plurality of protrusions is provided along a circumference of an outer ring valve seat 7. At low temperature, the disklike reverse bimetal 12 reverses to be convex upward, thus a peripheral rim comes into contact with the bimetal seat 14, and the center part rises to forcibly raise the valve disk 11 to open. At high temperature, it reverses to be convex downward, thereby causing the peripheral rim to release from the bimetal seat 14. While the disklike reverse bimetal 12 opens the valve disk 11 at a low temperature, a fluid flowing down on a lower surface of the valve disk 11 is likely to flow in above the disklike reverse bimetal 12 through a gap between lower protrusions of the peripheral rim of the disklike reverse bimetal 12, and thus the disklike reverse bimetal 12 can quickly reverse to be convex downward at high temperature.