To remarkably increase the overall strength by arranging expand metal sections and plate sections into a shape bent to the opposite sides in turn at positions on the circumference.

The width of the whole board is set to W, the length of a plate section A to be left as an original plate is set to La, the length of an expand metal section B to be formed into a mesh shape is set to Lb, and this board is manufactured as specified. Refractive angles at connection positions J between the plate sections A and the expand metal sections B are adjusted by a means such as a refracting machine, and a flat plate-like serrated irregular board Z is formed. The whole shape of the serrated irregular board Z can be formed into a flat plate shape having the prescribed thickness. The plate sections A and the expand metal sections B are formed into a repeated rib structure with no slit, thus the function can be remarkably improved as compared with the general expand metal, and its effect as ribs is large when the plate sections A are formed in parallel with each other in particular.