In a joint structure, a tube member having a polygonal cross-section can be joined to a flange surface by full penetration welding with good strength margin, without imparting unreasonable force to the weld portion. The joint structure comprises a polygonal cylindrical lower guide tube joined by butt welding to a surface of a middle flange having a through-hole of the same shape as the lower guide tube, wherein at the end of the lower guide tube, within the linear portions that constitute each of the sides of the lower guide tube, there exists a weld portion welded to the surface in at least one location and an unwelded portion located separately from the weld portion, and a level difference at least as large as a distance equivalent to the weld shrinkage of the weld portion is provided between the end positions of the weld portion of the unwelded portion.