Abstract: Provided are a mechanical tube fitting for preventing bending fracture of a tube member to be joined when subjected to bending deformation, and a simple tube fitting structure including that tube fitting. In the tube fitting, a clearance between a tube member (32) to be joined and an outer pipe (12) or an inner pipe (14) which is coaxially inserted in the outer pipe (12) and gives a space between its outer surface and the inner surface of the outer pipe (12), in which space the tube member (32) is inserted, is sealed with a sealing member disposed between the outer pipe (12) and the inner pipe (14), and the above-mentioned components are fixed by caulking. The outer pipe (12) is configured to extend outwardly in its axial direction than the ends of the inner pipe (14).