5d in valve casing 65 are contained vertically in order from below a spring holder 53 a return spring 52 a check valve 66 a balance piston valve 67 a relief valve casing 69 and a plunger 70 valve casing 65 is closed by a cover 61 a valve body 66 of the check valve comprises a conical portion 66c and a round bar 66d axially extending from the center of conical portion 66c at the lower end thereof is formed a seat for spring 52 and the base of conical portion 66c is upwardly biased by spring 52 retained by spring holder 53 at the axis of valve body 66 is open a communicating bore 66a at the upper end of round bar 66d is formed an orifice 66b communicating with communicating bore 66a round bar 66d is inserted into a through bore at a partition 67a to be discussed below and the upper end of the same upwardly projects from partition 67a balance piston valve 67 is pipe like shaped and forms therein partition 67a for partitioning a substantially vertically central portion of balance piston valve 67 communicating bores 67b are open at the outer periphery of balance piston valve 67 below partition 67a a first seat surface 67c is formed at the inner periphery of the lower end of balance piston valve 67 so that conical portion 66c of check valve 66 is brought into close contact with first seat surface 67c a conical portion 67d is formed at the outer periphery of the lower end of balance piston valve 67 a second seat surface 65a is formed in valve casing 65 and the upper end of balance piston valve 67 abuts against a spring 68 so that conical portion 67d is biased.