PURPOSE: To eliminate chattering of a ball valve body during pressure governing operation.

CONSTITUTION: An arc guide wall 7 for advance/retreat guiding a ball valve body 4 and an oil outlet 10 are formed extending over an advance/retreat movement range of the ball valve body 4 in an outlet communication chamber part 6b of a relief valve chamber. The guide wall 7 having the same diameter arc as the inside diameter of the valve chamber part 6a of the relief valve chamber is so constituted that the ball valve body 4 is guided to be closely fitted thereto so as to be advanced/retreated from the valve chamber part 6a to the outlet communication chamber part 6b.