PURPOSE: To suppress the generation of a distortion and to form a box-shaped panel in a short time by placing a circular arc-shaped member in a part which removed the corner part of a box-shaped panel body and joining and fixing the panel body and the circular arc-shaped member.

CONSTITUTION: The panel body 7 is formed integrally by means of bending, corner parts 5A, 5B are removed, a circular arc-shaped corner member 8 obtained by dividing a round bar 9 into four equal parts is placed in the removed parts, and the surface is finished by joining the corner member 8 and the body 7 by means of welding or brazing. Accordingly, comparing with that joined by welding over the whole surface of the panel as before, the generation of the distortion can be suppressed, the time required for straightening is eliminated, and the work time can be shortened.