To raise strength and rigidity as compared with a conventional welded T-joint even without reinforcing fitting.

An end of another pipe 7 mounted on a peripheral surface 13 of one pipe 6 so as to cross in a T-shape is enlarged, and fillet welded to the surface 13 of the one pipe 6 as a welding opposed pipe. Thus, a diameter of the end 14 of the one pipe 6 is increased to increase a strength of the pipe against bending in its direction and to increase its welding area, and the strength of the joint as a whole is raised. Even when the pipe having a relatively slender diameter is used, the strength of a frame structure can be raised.