To raise seat surface pressure when leakage is generated from a seat so as to suppress leakage by drawing a valve seat to a valve element or separating it from the valve element by operating fluid pressure.

In a seat part 17, a cylindrical annular projection 18 and a valve box 11 are integratedly formed around a passage 12 for a fluid, and an annular seat ring 21 of a U-shaped cross section is disposed as a valve seat for covering the annular projection 18. A sealed space 27 is formed between a second O-ring 26 and a first O-ring 25 disposed between the annular projection 18 and the seat ring 21. An operating fluid is supplied from a pressurizing and depressurizing device 29 into the sealed space 27, an inside of the sealed space 27 is set in a pressurizing condition, the seat ring 21 is drawn to a valve element 14 of a closing condition, and the valve element 14 is push-pressed. An inside of the sealed space 27 is set in a depressurizing condition, and the seat ring 21 is separated from the valve element of the closing condition.