PURPOSE: To perform machining of a second joint part from the outer peripheral side and to prevent the damage of the annular packing of a first joint part by effecting setting in such a manner that a through-oil passage is not overlapped with the annular packing of the first joint part.

CONSTITUTION: A plurality of the oil passages 13 and 13 on the middle cylinder side of a second joint part (b) comprises axial oil passages 13a and 13a formed along an axial direction formed in the cylinder body of a middle cylinder body 2; through-oil passages 13b and 13b extending through the outer peripheral surface of the muddle cylinder 2 toward the inner peripheral surface thereof ; and plug members 13c and 13c with which the outer end opening holes of the through-oil passages 13b and 13b are sealed. The through-oil passages 13b and 13b are formed in such a state not to be overlapped with the annular packings 5 and 5 of a first joint part (a). This constitution machines the oil passages 13 and 13 on the middle cylinder of the second joint part (b) from an outer peripheral surface and prevents the damage of the annular packings 5 and 5 of the first joint part (a).