PURPOSE: To improve workability by jointing the band plate of large width provided with folded sections at both the ends, and the band plate of small width, to each other, to form a frame base body, and by arranging an overhang rail fitting recessed section and a wall plate jointing recessed groove respectively on both the front and rear surface sides of the frame base body.

CONSTITUTION: A plane section and the V-groove surfaces of folded sections 5 on which the facing plate 2 of the band plate 1 of large width is not fitted, are coated with adhesives 6. After that, the facing material 4 of the band plate 3 of small width is jointed outside the plane section of the band plate 1, and the sections 5 to be folded are folded in order, and the short surfaces of the folded sections 5 are jointed to both the end surfaces of the band plate 3, and a frame base body is formed. After that, at both the upper surface end sections of the frame base body, wall plate fitting recessed grooves 8 are arranged. Then, on the almost central section of the lower surface of the frame base body, a rail fitting recessed section 7 is set. Besides, on non-machined sections generated at the corner sections of the recessed section 7, notched grooves 10 reaching the internal section of the band plate 3 along the recessed section 7 are arranged, and an upper frame body A is formed.