To keep the opening pressure of a relief valve at a constant value without being influenced by the liquid pressures of two chambers separated by a valve seat.

A first piston 22, a first spring 24, a second piston 27, and a second spring 31 are arranged in series in a cylinder 21. The second piston 27 is provided with pressure receiving surfaces 27f and 27g to receive a pressure such that a hydraulic pressure in a third chamber 28 communicated with a second chamber 26 is moved in a direction in which it is separated away from the first piston 22. Since, when a hydraulic pressure in the second chamber 26 is increased, the pressure receiving force in a closing direction of the first piston 22 is increased but since the energization forces in an opening direction of the first and second springs 24 and 31 on the first piston 22 are reduced, a force exerted in a closing direction on the first piston 22 is kept at a constant value. This constitution keeps the opening pressure of the first piston 22 at a constant value.