PURPOSE:To supply water under a constant low pressure even at a low place by allowing the forces acting onto the inclined surfaces formed onto a piston to offset each other even if a primary pressure is high. CONSTITUTION:A piston 3 is constituted of a head part 9, trunk part 10, and a lower part 11, and the outside diameter of the head part 9 is equal to that of the lower part 11, and the outside diameter of the trunk part 10 in the intermediate part is reduced, and each degree of inclination of a downward inclined part 12 and the upward inclined part 13 is constituted so that the symmetry in the vertical direction is established. The downward force acting onto the upper surface of the head part of the piston 3 and the force of a spring 19 are balanced by the secondary pressure generated in the case when a faucet is closed. Since the force due to the primary pressure in a water supply pipe is the sum of the force acting onto the inclined surface 12 and the force acting onto the inclined surface 13, and these forces offset each other, and the influence of these forces for other parts is eliminated.
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