To resolve air binding by small operating force of a bimetal ring.

A valve disk 11 is arranged in a variable pressure chamber 4 which is formed of inner and outer ring valve seats 6, 7 and a lid member 5. A bimetal ring 13 having ends which performs expansion-opening/contraction-closing operation by temperature change is arranged in the outer periphery of the outer ring valve seat 7. One end of the bimetal ring 13 is fixed to the outer periphery wall of the outer ring valve seat 7 with a screw 14. A plurality of communicating holes 15 which communicate an annular groove 8 between the inner ring valve seat 6 and the outer ring valve seat 7 and the variable pressure chamber 4 are formed in the outer ring valve seat 7 from one end to the other end of the bimetal ring 13. By the expansion-opening and contraction-closing action of the bimetal ring 13 according to the temperature change, the plurality of communicating holes 15 are opened sequentially during low temperature from the other end side, and are closed sequentially during high temperature from the one end side, and thereby the air binding is resolved by the small operating force of the bimetal ring 13.