PURPOSE: Firstly to eliminate need for welding, etc., and to facilitate production as the structural body is molded by slit forming, expanding and reinforcing part forming and secondly to obtain the structural body which is excellent in terms of strength as the structural body is integrally molded.

CONSTITUTION: Many slits 6 are formed zigzag in a part of the prescribed area of a prepd. flat plate 5 made of a metal and thereafter, this plate is expanded in a rectangular direction, by which the expanded metal part having many waveform shapes and many apertures of a uniform shape is formed. The prescribed reinforcing part is formed at least in a part of the remaining area of the flat plate 5 simultaneously therewith, by which the structural body is integrally molded.