A relief valve is provided with: a case; a valve seat provided in a flow passage; a poppet valve adapted to be seated on and lifted from the valve seat and opening and closing the flow passage; a spring having one end for pressing the poppet valve in the direction in which the poppet valve is seated on the valve seat; an adjustment screw engaged with the case through threads and changing, according to the depth to which the adjustment screw is screwed in, the distance between the valve seat and a seating surface with which the other end of the spring is in contact; and a positioning plate having an engagement hole engaging with the protrusion of the adjustment screw so that the adjustment screw is not relatively rotatable, the positioning plate being affixed to an end surface of the case and being in contact with the adjustment screw only at a side surface of the protrusion of the adjustment screw, preventing the rotation of the adjustment screw.