PURPOSE: To obtain high strength, to enable production at a large size, to decrease the number of stages and to enable mass production by forming wooden flat plates with adhesive layers of a chemical glue, etc., on both surfaces of an extrusion foamed flat plate which is a base material and forming end materials by adhesive layers in sectional parts.

CONSTITUTION: The flat plates 3 are formed by the adhesive layers 2 of the chemical glue, etc., on both surfaces of the extrusion foam 1 and the end materials 4 are formed with the adhesive layers 2 on the sections. A polystyrene board, veneer laminated plywood, PVC polywood, printed plywood, etc., are used for the flat plate 3. Consequently, the flat plate having any intricately curved surfaces is cut and the end materials are formed on the sections thereof with the adhesive layers. Even a change of field sizes varying from a design drawing is easily dealt with.