A pressure reducing valve comprising: a piston disposed in a cylinder chamber such that the piston is configured to reciprocate; a valve element that is coupled to the piston and is configured to open and close a valve seat, the valve element provided in, and having a diameter smaller than the diameter of, an inner cavity of the cylinder chamber; a valve spring that biases the valve element in a direction in which the valve element is moved away from the valve seat; and a biasing member that biases the valve seat toward a side opposite to the piston, wherein the pressure reducing valve is configured to reduce a primary pressure of a fluid to a secondary pressure, the fluid flowing from a primary pressure chamber into a secondary pressure chamber of the cylinder chamber through clearance between the valve element and the valve seat, the valve seat is disposed in the primary pressure chamber and the valve seat is configured to reciprocate in the same direction as a direction in which the valve element reciprocates, as a difference between the primary pressure and the secondary pressure increases, the valve seat moves toward the secondary pressure chamber, the valve element is disposed at a position closer to the secondary pressure chamber than the valve seat is, a flow path is provided in the piston and has one end that is open at an end face of the piston and has another end on the side of the valve seat that communicates with the inner cavity via another flow path extending perpendicularly to the axis of the cylinder chamber, and a primary port which is an inlet port through which the fluid flows into the pressure reducing valve, the inner cavity, and the flow path that is provided in the piston are provided coaxially to the axis of the cylinder portion, characterized in that the pressure reducing valve comprises a valve seat holder configured to hold the valve seat and configured to be movable together with the valve seat, wherein the valve seat holder has a cylindrical portion having a communication hole that is coaxial with the axis of the cylinder chamber and communicates with the primary pressure chamber, and a flow path connected to a secondary port for the fluid from the pressure reducing valve is disposed coaxially to the axis of the cylinder chamber. a piston disposed in a cylinder chamber such that the piston is configured to reciprocate; a valve element that is coupled to the piston and is configured to open and close a valve seat, the valve element provided in, and having a diameter smaller than the diameter of, an inner cavity of the cylinder chamber; a valve spring that biases the valve element in a direction in which the valve element is moved away from the valve seat; and a biasing member that biases the valve seat toward a side opposite to the piston, wherein the pressure reducing valve is configured to reduce a primary pressure of a fluid to a secondary pressure, the fluid flowing from a primary pressure chamber into a secondary pressure chamber of the cylinder chamber through clearance between the valve element and the valve seat, the valve seat is disposed in the primary pressure chamber and the valve seat is configured to reciprocate in the same direction as a direction in which the valve element reciprocates, as a difference between the primary pressure and the secondary pressure increases, the valve seat moves toward the secondary pressure chamber, the valve element is disposed at a position closer to the secondary pressure chamber than the valve seat is, a flow path is provided in the piston and has one end that is open at an end face of the piston and has another end on the side of the valve seat that communicates with the inner cavity via another flow path extending perpendicularly to the axis of the cylinder chamber, and a primary port which is an inlet port through which the fluid flows into the pressure reducing valve, the inner cavity, and the flow path that is provided in the piston are provided coaxially to the axis of the cylinder portion, characterized in that the pressure reducing valve comprises a valve seat holder configured to hold the valve seat and configured to be movable together with the valve seat, wherein the valve seat holder has a cylindrical portion having a communication hole that is coaxial with the axis of the cylinder chamber and communicates with the primary pressure chamber, and a flow path connected to a secondary port for the fluid from the pressure reducing valve is disposed coaxially to the axis of the cylinder chamber.