PURPOSE: To manufacture a symmetrical article, without the need of a large- scale working equipment, for a reduced time in bending, under an easy parts control in the manufacturing processes, and with a superior shape accuracy.

CONSTITUTION: A notch 3 is formed in the center in the longitudinal direction of a work 2 of an aluminum hollow extruded material, and the work is folded over at the notched part 3, in which state a two-dimensional bending is performed, on both sides of the work parts 7, 9 holding the folded part of the work 2 in between, integrally within a plane vertically crossing in the parallel direction of the work part; after that, the work 2 is folded back at the folding part so as to make notched faces 10, 11 abutted on each other, and welding 12 is performed.