To provide technology by which a pipe structure can be easily and highly precisely composed eliminating restriction against shape precision in a rectangular pipe frame with a joining face subjected to press forming.

One curved part 11 of a rectangular pipe frame 10 to be press-formed is provided with projecting parts 12, 13, and 14, and the corresponding positions in the other curved part 15 are provided with recessed parts 16, 17, and 18. Holes 19 and 20 and an embossed part 21 of required shapes are formed on the projecting parts 12, 13, and 14 in the curved part 11. Both the curved parts 11 and 12 are butted, and the projecting parts 12, 13, and 14 are inserted into the recessed parts 16, 17, and 18, so that the structure as that of a rectangular pipe frame is made. Since the holes 19 and 20 and embossed part 21 are previously formed on either curved part 11, it can be arranged in a good shape and a good position, and fitting precision on use, e.g., as the positioning holes or tapped holes of a fitting increases.