A rolled steel plate 1 wound in a roll shape is unrolled and is cut in given dimensions to form a blank 2. In this case, a cut surface 2a is caused to incline at 1 to 3 degrees relative to the thickness direction of the blank 2, wherein the inclination direction of the cut surfaces 2a, 2a on each side is reversed. The blank 2 cut in this manner is roll-formed to cause the cut surfaces 2a, 2a to confront each other. Since the cut surfaces 2a, 2a on each side incline at 1 to 3 degrees in the opposite direction as described above, the cut surfaces 2a, 2a are caused to be parallel. Then, the confronting sections are laser-welded to form a cylindrical body 3.