PURPOSE: To facilitate welding and as well to reduce strain, by fillet-welding a pair of inner structure members with a vertical bracket being disposed between the opposed ends thereof.

CONSTITUTION: The opposed joint faces of a pair of inner structure members 4, 5 are positioned in the center position of a frame 6, and the joint faces thereof are made straight with no included angle. A bracket 7 is stood upright on the frame 6, and is held between the inner structure members 4, 5, and the inner members 4, 5 and the bracket 7 are fillet-welded together from four sides (c, d, e, f).