To reduce the back pressure during a relief operation, prevent fluctuation in a relief setting pressure, and thereby realize a stabilized relief operation.

A valve guide 34 for accommodating a relief valve body 18 is formed into a cylindrical shape in which the dimensions of the outside diameter are substantially equal from one end side to the other end side. Thus, the capacity of an oil chamber 35 in which relief oil circulates during relief operation can be enlarged to prevent generation of eddy flows during the relief operation. On the other hand, an opening 41A which is gradually enlarged on the outer periphery side of the valve guide 34 is formed in a communicating hole 41 of an accumulator 37 to prevent an O-ring 42 of a piston 40 from being damaged during the period of manufacturing a relief valve 31 with an accumulator.