To reduce the warpage and to improve the biting of adhesive by molding sand embossing on the coated surface of waterproof paint formed on the rear surface side.

The upper surface of an intermediate specific gravity fiber plate 10 is coated with waterproof paint 11 made of two-pack curable polyurethane paint. The paint is dried and solidified, and then inverted. The rear surface is coated with adhesive 11, and a decorative sheet 13 made of printing decorative sheet is laminated. A mirror embossing platen 14 is hot pressed to the sheet 13 surface and the paint 11 surface is hot pressed by a hot pressing machine for urging an embossing platen 15 having sand embossing. Thus, warpage or dimensional error is reduced, and the adhesive properties of the adhesive to the rear surface side is improved.