To provide a pressure reducing valve which can prevent increase in secondary pressure due to lowering of water cutoff effect, which is caused by deformation of a piece packing for closing a valve seat when the packing surface is contaminated, foreign matter adheres to the packing surface, or the packing is pressed to the valve seat by excessive force.

A flange part having a fixed length and a diameter a little smaller than the inside diameter of a valve seat is formed in a part of a piston where the upstream end part of a piece packing abuts, whereby even in a small flow range, the valve piece packing is kept away from the valve seat as much as possible. Further, in the valve piece connected to the piston, an O-ring is used as a check valve so that when the primary pressure on the inflow port side is higher than the secondary pressure on the outflow port side, pressure water from the inflow port is cut off, and in the reverse case, the pressure water on the outflow side is let escape to the inflow port side, thereby preventing increase in the secondary side pressure caused by the piece packing.