A relief valve, comprising: a housing including a valve passage having an inlet port through which an operating oil flows in and an outlet port through which the operating oil flows out; a plunger housed in the housing in a manner to be movable to each of a closed position and an open position and pressed toward the open position by the operating oil in the inlet port, the closed position being a position in which a valve portion formed at one end of the plunger is seated on a valve seat of the housing to close the valve passage, the open position being a position in which the valve portion is off the valve seat to open the valve passage; a differential pressure chamber into which the operating oil in the inlet port is guided via a through passage formed in the plunger and which presses an other end of the plunger toward the closed position by the operating oil guided; a spring member that biases the plunger toward the closed position; a piston exteriorly provided in a manner to be movable along the plunger and configured to compress the spring member by moving according to a pressure of the operating oil that is guided to the differential pressure chamber; and a damping chamber formed around an outer peripheral surface of the piston and configured to store the operating oil to restrict movement of the piston in a compression direction, wherein: the plunger includes a first damping passage penetrating the plunger and bringing an area outside the plunger and the through passage into communication; the piston includes a second damping passage penetrating the piston and bringing an area inside the piston and the damping chamber into communication; and a communication space connected to each of the first damping passage and the second damping passage is formed between the plunger and the piston. a housing including a valve passage having an inlet port through which an operating oil flows in and an outlet port through which the operating oil flows out; a plunger housed in the housing in a manner to be movable to each of a closed position and an open position and pressed toward the open position by the operating oil in the inlet port, the closed position being a position in which a valve portion formed at one end of the plunger is seated on a valve seat of the housing to close the valve passage, the open position being a position in which the valve portion is off the valve seat to open the valve passage; a differential pressure chamber into which the operating oil in the inlet port is guided via a through passage formed in the plunger and which presses an other end of the plunger toward the closed position by the operating oil guided; a spring member that biases the plunger toward the closed position; a piston exteriorly provided in a manner to be movable along the plunger and configured to compress the spring member by moving according to a pressure of the operating oil that is guided to the differential pressure chamber; and a damping chamber formed around an outer peripheral surface of the piston and configured to store the operating oil to restrict movement of the piston in a compression direction, wherein: the plunger includes a first damping passage penetrating the plunger and bringing an area outside the plunger and the through passage into communication; the piston includes a second damping passage penetrating the piston and bringing an area inside the piston and the damping chamber into communication; and a communication space connected to each of the first damping passage and the second damping passage is formed between the plunger and the piston. the plunger includes a first damping passage penetrating the plunger and bringing an area outside the plunger and the through passage into communication; the piston includes a second damping passage penetrating the piston and bringing an area inside the piston and the damping chamber into communication; and a communication space connected to each of the first damping passage and the second damping passage is formed between the plunger and the piston.