A flow-rate adjustable valve for adjusting a flow rate of liquid flowing through a flow path, the flow-rate adjustable valve comprising: a main valve body movably supported in the flow path via a diaphragm membrane, a back pressure chamber in which liquid supplied from an upstream side of the flow path at a predetermined pressure is contained and a biasing force is generated in a valve-closing direction for closing the main valve body by the supplied liquid, a plurality of inlet holes communicating the upstream side of the flow path with the back pressure chamber, an outlet hole communicating a downstream side of the flow path with the back pressure chamber, a pilot valve body for opening and closing the outlet hole, and an elongated member for holding the pilot valve body and causing the pilot valve body to move in an axial direction of the elongated member, characterized in that: moments acting on the main valve body based on the liquid flowing into the back pressure chamber through the plurality of inlet holes are canceled by each other in total, under a condition in which the pilot valve body opens the outlet hole, the elongated member comprises a distal-end member holding the pilot valve body and a shaft member connected to the distal-end member, the shaft member extending continuously both through a region on the side of the back pressure chamber, in which the liquid can exist, and through an atmospheric region, in which the liquid cannot exits, via a water-tight seal, and a cross-sectional area of the shaft member at the part passing through the water-tight seal is configured to be smaller than an opened area of an end of the outlet hole on the side of the back pressure chamber. a main valve body movably supported in the flow path via a diaphragm membrane, a back pressure chamber in which liquid supplied from an upstream side of the flow path at a predetermined pressure is contained and a biasing force is generated in a valve-closing direction for closing the main valve body by the supplied liquid, a plurality of inlet holes communicating the upstream side of the flow path with the back pressure chamber, an outlet hole communicating a downstream side of the flow path with the back pressure chamber, a pilot valve body for opening and closing the outlet hole, and an elongated member for holding the pilot valve body and causing the pilot valve body to move in an axial direction of the elongated member, characterized in that: moments acting on the main valve body based on the liquid flowing into the back pressure chamber through the plurality of inlet holes are canceled by each other in total, under a condition in which the pilot valve body opens the outlet hole, the elongated member comprises a distal-end member holding the pilot valve body and a shaft member connected to the distal-end member, the shaft member extending continuously both through a region on the side of the back pressure chamber, in which the liquid can exist, and through an atmospheric region, in which the liquid cannot exits, via a water-tight seal, and a cross-sectional area of the shaft member at the part passing through the water-tight seal is configured to be smaller than an opened area of an end of the outlet hole on the side of the back pressure chamber. moments acting on the main valve body based on the liquid flowing into the back pressure chamber through the plurality of inlet holes are canceled by each other in total, under a condition in which the pilot valve body opens the outlet hole, the elongated member comprises a distal-end member holding the pilot valve body and a shaft member connected to the distal-end member, the shaft member extending continuously both through a region on the side of the back pressure chamber, in which the liquid can exist, and through an atmospheric region, in which the liquid cannot exits, via a water-tight seal, and a cross-sectional area of the shaft member at the part passing through the water-tight seal is configured to be smaller than an opened area of an end of the outlet hole on the side of the back pressure chamber.