PURPOSE: To improve the quality in the external appearance of a product by making the corner ridge line of a recessed joint part a folding edge, forming an about U-shaped cutting edge at this place as well and folding the folding piece surrounded by the cutting edged by the corner ridge line.

CONSTITUTION: The corner ridge line (a) of the recessed joint 2 part of a corner main body 1 is taken as an edge of a foil. The cutting edge 3 in about U-shape communicating to the corner main body 1 at this place and being cut off at the other place is formed by penetrating into the main body 1. This plate-like intermediate material (f) is bent by the corner ridge line (a), the bending is executed at the place of a dot line (e) as well and the recessed joint piece 2 recessing at one step more than the main body 1 is formed. The bend piece 4 surrounded by the cutting edge 3 is bent by the ridge line (a) and arranged in opposition to the back face of an adjacent recessed joint piece 2a. The bending formation of the recessed joint 2 is thus facilitated and the external appearance of the product bending line, etc., is improved. Consequently the external appearance quality of a product is improved.