PURPOSE: To reduce the elastic repulsion of a return spring by preventing the atmospheric pressure from working on a bent part by a retainer.

CONSTITUTION: A valve body 18 has a bent part 18b, and a retainer 20 has a wall part 20c and a cylinder part 20d extending into the inside of the bent part. A first sealed chamber 32 is formed, by forming a first sliding sealing part 18d which comes into sliding contact with the outer circumference of the cylinder part of the retainer on the inner surface of the bent part, and a second sealed chamber 33 is formed, by forming a second sliding sealing part 18e coming in sliding contact with the inner circumference of the valve body on the circumference of a placing part 18c of the valve body. A communicating hole 18f connects the first sealed chamber to the second sealed chamber, a communicating hole 18g connects the second sealed chamber to a variable pressure path 25. If may be allowed to omit the second sliding sealing part 18e, the second sealed chamber 33, and the communicating hole 18g, so that the first sealed chamber can be connected to a constant pressure path 22.