PURPOSE: To enlarge the contact surface of a joint part, to improve the mechanical strength and to obtain good appearance without cracks in the periphery by providing a tube internal end part with an axial slotted cylindrical section integrally formed with a concave end wall.

CONSTITUTION: A reinforcement element 10 is formed by side walls 12a, b and an end wall 14 integraly formed therewith to have the same form as that of the inner surface of an open end of a tube member 12, and the end wall 14 is formed in the same shape as that of the outer surface of a tube member 4. Accordingly, the contact surface can be enlarged to obtain high mechanical strength, the appearance can made favorable without cracks, and working can be simplified.