A pressure reducing valve comprising: an open-close valve that switches between connecting and isolating a primary port and a secondary port provided in a body member; a cylinder arranged downstream of the open-close valve, the cylinder communicating with the secondary port; and a piston that divides the cylinder into a pressure reducing chamber and a pressure regulating chamber, wherein the piston moves inside the cylinder according to a difference between force applied from the pressure regulating chamber and force applied from the pressure reducing chamber, thereby opening and closing the open-close valve, the open-close valve includes a valve stem, which is located between the open-close valve and the piston, and a plug, which restrains displacement of the piston toward an upstream side of the open-close valve by abutting on a lower end surface of the piston, the plug houses the valve stem and includes four radially extending channel holes through which a housing part for the valve stem communicates with the pressure reducing chamber, an upper part of the plug is hexagonal and functions as a tool engagement part provided at a downstream end portion of the plug, the tool engagement part has corners, the tool engagement part is arranged around an axis of the plug, a first screw part is formed in the body member, a second screw part, which is threaded to the first screw part, is formed in the plug, while the first screw part is being threaded to the second screw part, a tool can engage the tool engagement part, thereby attaching the plug to the body member, the channel holes are formed in the hexagonal upper part in positions determined so that the corners do not intersect with the channel holes, the channel holes being arranged at equal angular intervals about the axis of the plug, each channel hole is formed in one of the faces of the hexagonal upper part, an upper end surface of the plug, which is an end surface of the hexagonal upper part, and the lower end surface of the piston come into surface-contact with each other, the valve stem includes multiple channel grooves extending in the same direction as an axis of the plug, and the number of the channel holes is the same as that of the channel grooves, and the channel grooves are spaced apart at equal angular intervals about an axis of the valve stem. an open-close valve that switches between connecting and isolating a primary port and a secondary port provided in a body member; a cylinder arranged downstream of the open-close valve, the cylinder communicating with the secondary port; and a piston that divides the cylinder into a pressure reducing chamber and a pressure regulating chamber, wherein the piston moves inside the cylinder according to a difference between force applied from the pressure regulating chamber and force applied from the pressure reducing chamber, thereby opening and closing the open-close valve, the open-close valve includes a valve stem, which is located between the open-close valve and the piston, and a plug, which restrains displacement of the piston toward an upstream side of the open-close valve by abutting on a lower end surface of the piston, the plug houses the valve stem and includes four radially extending channel holes through which a housing part for the valve stem communicates with the pressure reducing chamber, an upper part of the plug is hexagonal and functions as a tool engagement part provided at a downstream end portion of the plug, the tool engagement part has corners, the tool engagement part is arranged around an axis of the plug, a first screw part is formed in the body member, a second screw part, which is threaded to the first screw part, is formed in the plug, while the first screw part is being threaded to the second screw part, a tool can engage the tool engagement part, thereby attaching the plug to the body member, the channel holes are formed in the hexagonal upper part in positions determined so that the corners do not intersect with the channel holes, the channel holes being arranged at equal angular intervals about the axis of the plug, each channel hole is formed in one of the faces of the hexagonal upper part, an upper end surface of the plug, which is an end surface of the hexagonal upper part, and the lower end surface of the piston come into surface-contact with each other, the valve stem includes multiple channel grooves extending in the same direction as an axis of the plug, and the number of the channel holes is the same as that of the channel grooves, and the channel grooves are spaced apart at equal angular intervals about an axis of the valve stem.