PURPOSE: To remarkably facilitate the repairs of a flush valve for supplying cleaning water into a water closet or the like, by using a new pilot hole which is opened by turning a plate with the use of a tool or the like when the a pilot small hole in a piston main valve clogs.

CONSTITUTION: A push button is depressed so as to tilt a start valve 24 through the intermediary of a push-rod 28 so that the valve 24 is opened, water pressure is discharged from a pilot small chamber B through a pilot small hole 44, and accordingly, a piston main valve 16 is pushed up by a supply water pressure in an inflow side chamber A so as to be opened, resulting discharge of water into a closet from an outlet port 14. In this arrangement, if the pilot small hole 44 clogs with foreign matter, a stopper 32 is turned and removed after the supply of water is stopped, and a drive is inserted into a stopper screw insertion hole 31 which has been opened. The tip end of the driver is fitted in a groove 68 formed in the boss 62 of a plate 60 which is therefore turned in order to align an aperture 70 with another pilot hole 44 which can be therefore used.