PURPOSE: To enhance surface hardness, nail-holding force, bending strength, water resistance of surface and the like, by a method wherein a composition for forming a foamed body of a synthetic resin is coated on a surface part of a fibrous plate, and the composition is brought into reaction while pressing the surface part.

CONSTITUTION: A foamable synthetic resin liquid composition containing a synthetic resin such as a styrene resin, an acrylic resin, a phenolic resin, etc. and a foaming agent which generates a decomposition gas under heating, is prepared. For example, a foamable synthetic resin liquid composition containing 100pts.wt. of a phenolic resin and 3W5pts.wt. of toluenesulfonyl hydrazide as the foaming agent, is prepared. After coating the resin liquid composition on the surface of a wood fibrous plate having an intermediate or low specific gravity such as a soft fibrous plate, the liquid composition is uniformly spread by pressing by a roller or the like as required to impregnate the resin liquid into the surface part of the fibrous plate. Then, the liquid composition is brought into reaction while pressing the fibrous plate by a pressing device such as a flat-plate press to foam the composition.