PURPOSE: To prevent the warping of a molding when swelled by a method in which cellulosic short fibers are pressed temporarily to form a pressed board, an adhesive is coated on the surface of the pressed board, and the pressed board is sufficiently pressed for molding.

CONSTITUTION: Cellulosic base material (e.g., wood chips, pulp, etc.) is dried impregnated with an urea resin liquid within 5W20°/wt%. The base material so treated is allowed to stand for 20W30min and dried by heating to form an aggregate 1. A material formed by mixing the aggregate 1 and fibers 2 or the material with a net cloth 3 is temporarily hot- or cold-pressed to cause the integral adherence of the aggregates 1, the entanglement of the fibers 2 and the net cloth 3. Since given heat-insulating spaces are formed among the aggregates 1 and swelling and absorption spaces for the swelling, the aggregate 1 are formed in the raising portions 1a, the heat-insulating effects of the synthetic wooden board can be enhanced.