To rationally design a steel pipe to be used as a skeleton of a specific structure in which a direction where a bending moment acts greatly is determined or a portion to promote corrosion is determined, and to reduce costs as a result.

An unequal thickness steel pipe 1 is used which comprises a thin cross section 11 which is provided in a part of a pipe circumferential direction, and a thick cross section 13 which is provided in a portion different from the thin cross section 11 in the pipe circumferential direction and is formed thicker than the thin cross section 11. When installing the unequal thickness steel pipe 1 is in a structure 3 as a skeleton 31, it is installed in such a manner that the thick cross section 13 is located in a predetermined portion in accordance with an install position.