PURPOSE: To eliminate the waste of a material besides to have sufficient strength and to lighten inweight by combining the plural ribs separately formed on the back face of the top plate formed in about flat shape by calking, etc., at intervals in parallel each other.

CONSTITUTION: The combination of each rib 6 for the back face of a top plate 5 is to reverse the top plate 5 upside down and to mount the rib 6 one by one by reversing it on the position mutually between a pair each of the projecting bar parts 5a arranged in parallel at an interval (a). In this state it is fed among the die 7 arranged at the lower side of a follower plate locking device and the punch 8 arranged at the upper side and a plate presser 9, and the top plate 5 and the both side flange parts 6b of the rib 6 superposed thereon are interposed from the upper and lower parts among them. The punch then pushed in by hydraulic driving, etc. to combine the top plate 5 and flange part 6b by calking 10 each other. Consequently sufficient strength is secured to eliminate the waste of materials.