Abstract: A metal pipe 11 of a smaller diameter engages a metal pipe 13 of a larger diameter to provide a double-pipe structure in a predetermined area in the longitudinal direction, wherein the diameter of the two pipes 11 and 13 of the double-pipe structure section is integrally expanded using a pipe expansion technique. By making a predetermined section of the pipe member a double-pipe structure, it is possible to provide the strength and rigidity necessary for each section. It is also possible to join the two pipes by expanding the diameter of each pipe at the double-pipe structure section. By forming a section where high strength is not required by a pipe of a smaller diameter, it is possible to lighten the pipe member. Materials for the pipes 11 and 13 are carbon steels for machine structural use. If aluminum is used for the metal pipe 11 of a smaller diameter, it is possible to further lighten the pipe member.