To prevent the leakage of the pressurized fluid from contact surfaces while maintaining the even required surface pressure over the whole contact surface by forming one of contact surfaces of a casing and a cover so that a part thereof inside a fastening bolt is projected from an outside part toward a mating surface by a constant quantity.

Plural bolt holes 11a for fastening a cover 11 to a valve casing are drilled in the cover 11, and a lower opening part of the bolt hole 11a as a contact surface 15 contacts with the valve casing. The contact surface 15 is formed so that an end surface of a cylindrical projecting part 21b is finished smooth and so that an inside contact surface 12 inside of the bolt hole 11a for insertion of fastening bolt is projected by the predetermined quantity from the outside contact surface 13 outside of the bolt hole 11a. Quantity of this projection is desirably set to about 5/100 mm. Even in the case where unevenness of contact surface pressure is generated in the outside part and a part at a pressure less than the predetermined surface pressure is generated, high surface pressure can be held over the whole surface of the inside part so as to surely prevent the leakage of the pressurized fluid.