To provide a manufacturing method where the butted parts of a plurality of extrusion shape materials are welded by friction stir welding, so as to make a first structure, and next, second structures are welded to the first structure, the width of the first structure after the completion is made the prescribed one over the whole length.

A stand frame 10 as a first structure is composed in such a manner that a plurality of extrusion shape materials 20 are arranged in the width direction of the stand frame 10, and the mutual butted parts are subjected to friction stir welding. Thereafter, bolsters 30 as second structures are arranged at the back face of the stand frame 10 orthogonally to the longitudinal direction thereof, and are welded so as to be fixed to the stand frame 10. At the time of the friction stir welding, the moving speed of a rotary tool is made low at the positions where each bolster 30 is to be fixed and at their vicinities, and the increase in the temperature of the stand frame 10 at each part is suppressed, thus the width of the stand frame in each part after cooling is increased compared with the other part. Thus, even in the case the width of the stand frame in each part is thermally shrunk after the welding of the bolsters 30, each part fits in the size same as that of the other part.