A pressure reducing valve has a seal member, which is mounted on a side wall of a piston. The seal member is arranged between a first ring and a second ring. Consequently, depending on the direction of displacement of the piston, the first ring or the second ring is positioned on one of a displacement direction upstream side or a displacement direction downstream side of the seal member. The first ring and the second ring are made of a resin material, for example a polytetrafluoroethylene resin or a polyetheretherketone resin, having lower coefficient of friction compared with that of the seal member.