PURPOSE: To prevent the deformation of and damage on a sealing ring by providing an annular wall at the bottom of an operation water chamber and arranging the sealing ring with a vertical U-shape cross-section between the annular wall and the operation water chamber.

CONSTITUTION: An operation water supply mechanism 14 supplies an operation water to a closed chamber 13 formed between a piston slider 12 and a bowl 2, and presses the slider 12 to the lower surface of the periphery of a bowl hood 3 with the centrifugal water pressure of the operation water to close a discharge hole 11. An operation water discharge hole 15 discharges the operation water supplied to the closed chamber 13 to an area outside the bowl 2. In addition, the annular operation water chamber 19 pushes up a closing ring 17 with the centrifugal water pressure of the operation water through a ring piston 18 to close the operation water discharge hole 15 with a valve 16. Then an annular wall 21 is provided at the bottom of the operation water chamber 19 and a seal ring 20 with a vertical U-shape cross-section is arranged between the annular wall 21 and the outer peripheral wall of the operation water chamber 19. Consequently, the deformation of and damage on the sealing ring can be prevented.