To eliminate a gap G even without use of a spacer.

The end surface 61 of a diameter-enlarged portion 12 of a reinforcement 60 with a diameter-enlarged portion is pre-inclined at an angle θ of inclination with respect to a plane orthogonal to an axial center; the two reinforcements 60 and 60 are longitudinally joined together in such a manner that end surfaces 61 and 61 face each other in parallel; the diameter-enlarged portions 12 of them are housed in a hollow large-diameter portion, so that a joint member 40 for the reinforcement can be attached in such a manner as to cover the periphery; the connection of the joint member 40 is fixed; and subsequently, the end surfaces 61 and 61 are stretched by axially rotating the reinforcement 60, so that loosening in the direction of the axial center can be prevented by regulating the displacement of the diameter-enlarged portions 12 and 12 in the direction of the axial center.