A wide plate-like material (27), having thin and thick wall portions in a transverse direction, is prepared by alternately joining one of flat thin plates (28, 29) and one of thick corner plates (30) or thick plates (31). The wide plate-like material (27) is bent at and along right and left thick corner plates (30) and formed into a U-shaped structure (32) which is U-shape in cross section. Separately from the U-shaped structure (32), a plate-like member (33) is prepared by butt welding thick plates (35) to right and left sides of a thin plate (34). The plate-like member (33) is welded to a lower side of the U-shaped structure (32) by laser welding or the like to form a square tubular structure, which is in a square shape in cross section and serves as a major part of an operating arm.