To ensure connecting strength and low cost by additionally providing connecting edges almost at a right angle on the same side of two side edges, connecting the connecting edges of perforated plates of the same length, and providing a wide earth plate across the lower end parts of the perforated surface board body.

Three perforated plates 4 are erected at an included angle of about 60° to an earth plate 6, along the edge lacking a reinforcing edge 61 of the earth plate 6. Connecting edges 43 of the perforated plates 4 and connecting holes installed at the edge of the earth plate 6 are adjusted to each other and jointed by bolts and nuts to build up a surface frame body 3. The perforated plates 4 are also connected to one another by bolts and nuts, and a bracing plate 32 is held between to hold the angle of the perforated plate 4 and earth plate 6. In this case, the surface frame body 3 is built up being connected by bolts and nuts so as to be suitably built up near a construction site, and the perforated plate 4 high in cost in hand machining can be obtained by a steel plate pressing machine used to manufacture small parts of general steel plate building material by dividing the perforated plate 4 in a compact state. Productivity is therefore improved by a large margin, and cost can be reduced.