To prevent foreign matter from accumulating between an outer ring valve seat and a valve disk, and to solve air binding by lifting the valve disk with the small operating physical force of a bimetallic ring.

A conical bevel part 12 is formed on the periphery of the outer ring valve seat 7, the upper end and lower end of the bevel part 12 are respectively formed with an upper plane part 15 and a lower plane part 16, the ended bimetallic ring 13 is arranged, and an ended interposing ring 14 with the upper surface thereof obliquely formed is disposed between the bimetallic ring 13 and the valve disk 11. The bimetallic ring 13 is contracted at low temperature so as to be upwardly moved along the bevel part 12, swings and lifts the valve disk 11 with a single point at the uppermost end of the interposing ring 14 as a point of application of force to separate the valve disk from inner/outer ring valve seats 6, 7, and is placed on the upper plane part 15.