To facilitate manufacture by extending the whole body in the external direction, after welding across adjoining steel materials into a zigzag state at spots in both polymerized faces formed by laminating band steel materials.

A band steel 1 is folded at every necessary lengths, compacted. and laminated into a prescribed number of layers. Secondly, a zigzag welding points 2 are formed at every prescribed intervals across adjoining steel materials 1 in the front/rear both polymerized faces. Thirdly, a fixing jig 3 and a movable jig 4 are mounted on both ends of the steel material 1 and the whole body is extended so as to be into a set grating. A side frame plate and an edge frame plate are attached in the circumferential edge of the grating as necessary. The steel material 1 is used in such a state as being cut beforehand into a prescribed dimension or folded into two at a prescribed length so as to be into a U shape as necessary. This constitution can eliminate cutting so as to easily manufacture it in a small-scale facility without any necessity of sound control and dust collecting measures, reduce the cost, and improve rigidity and antislipping effects.