To prevent hydraulic oil from leaking out of the connected portion between a cylindrical plate and a cylinder to the outside when the cylinder is slid on the cylindrical plate.

The head part 40A of a cylinder 40 is formed in a generally semi-spherical shape having a spherical center point 50c on the center axis 40c of the cylinder 40. The portion 20A of the cylindrical plate 20 corresponding to the head part 40A is formed in a recessed shape matching the generally semi-spherical shape. Consequently, a spherical joint 50 can be formed. The cylinder 40 is connected, through the spherical joint 50, to the cylindrical plate 20 swingably around the spherical center point 50c. A seal member 51 is installed between the spherical surface 40A and the recessed surface 20A of the spherical joint 50. The side portion 40B of the cylinder 40 is formed in a spherical shape having the same spherical center point 50c as the spherical center point of the spherical joint 50. The portion 6 thereof corresponding to the cylinder side portion 40B is formed in a recessed shape matching the spherical shape. Therefore, a holding means 60 is formed. The cylinder 40 is swingably held on the cylindrical plate 20 through the holding means 60.