PURPOSE: To reduce the weight of a hose by imparting a resilience to the rear end of a piston in the valve closing direction, making the total pressure receiving area in the valve opening direction of a piston valve body larger than that in the vale closing direction thereof and providing a resisting plate in a passage in communication with a valve hole.

CONSTITUTION: When spraying pressure water, an adjusting spring 22 is adjusted by an adjusting screw rod 20 to impart forward motion to a piston 16 and close a valve hole 9 with a valve body 18. When terminating the spraying, the inner pressure of a main valve body 1 rises, and the pressure exerted on the piston 16 and the valve body 18 in the valve opening direction deflects an adjusting spring 22 acting on the piston 16 to move back the valve body 18 and open the valve hole 9. The pressure water then flows out from the valve hole 9 and, from a resisting plate 11 at the end of a circulating port 10, it is circulated back in a power-driven sprayer through a coupling 13 and a circulating pipe 12. At this time, although the inner pressure of the main valve body 1 is reduced, it will not fall below a predetermined value by the action of the resisting 11 with the valve hole 9 maintained in an open state. Upon restarting of the spraying, since the inner pressure of the main valve body 1 becomes even lower, the valve hole 9 is closed.