Insertion holes (3c, 4e) for inserting a connecting pipe are formed in such a manner as to extend through a casing (3) into a cylinder (4) to receive a connecting pipe (9) having a flange (9a) at a predetermined distance from its distal end. While this flange (9a) is kept in contact with the outer wall surface of the casing (3), the distal end portion of the connecting pipe (9) is inserted into the insertion holes (3c, 4e). An O-ring (01) is fitted between the flange (9a) and the outer wall of the casing (3) in such a manner as to encompass an internal passage (9f) of the connecting pipe (9). Another O-ring (02) is fitted between the distal end portion of the connecting pipe (9) and the insertion hole (4e) of the cylinder (4).
