To prevent trouble of disengagement of first and second abutment ends 11, 12 at one or both ends of a seal ring 9 from a seal ring groove 5 of a valve 3 and coming out.

A flat surface part 71 engaged with end surfaces of first and second engagement projection parts 51, 52 of the seal ring 9 is provided on a part in a valve circumferential direction of a groove bottom surface formation part 7 of the valve 3. The flat surface part 71 is abutted on end surfaces of first and second fitting projection parts 51, 52 of the seal ring 9 to rotation-stop the seal ring 9. Thereby, even if opening/closing action of the valve 3 is performed, the seal ring 9 is not deviated in the valve circumferential direction. Accordingly, since the state that the both first and second abutment ends 11, 12 of the seal ring 9 are restricted between a nozzle 2 and the groove bottom surface formation part 7 of the valve 3 can be maintained for a long period of time, the trouble in which one or both of first and second abutment ends 11, 12 of the seal ring 9 disengage from the seal ring groove 5 of the valve 3 and come out can be certainly prevented.