PURPOSE: To improve the extent of bending strength by forming more than two plates set up so as to be opposed, and connecting these more than two plates with more than two rib plates, thereby forming an integral panel.

CONSTITUTION: Both first and second plates 11 and 12 are opposedly set up via a gap G, and plural units of rib plates 13 extending in the extruding direction are formed at an interval so as to connect, these first and second plates 11 and 12. In addition, each angle between both these first and second plates 11, 12 and the rib plates 3 should be set to 90°, and each of rectangular space parts 14 is partitioned off, forming then so as to vary an extending direction of each rib plate 13. Accordingly, such directional uniformity as excellent in bending strength is thus securable.