PURPOSE: To prevent a decorative veneer and joining area from generating a crack and a gap and pushing-up, by a method wherein isocyanate is infiltrated into the decorative veneer, a polymer is formed and solidified by reacting the isocyanate and water to each other.

CONSTITUTION: A Japanese oak veneer is adopted as a decorative veneer, the decorative veneer having normally water content of 5-20% is dried, to begin with, and the decorative veneer is either immersed into an isocyanate solution obtained by diluting isocyanate with methanol or the isocyanate solution is cast into decorative veneer under a vacuous state. Then the decorative veneer into which the isocyanate solution is infiltrated is immersed into water, the isocyanate and the water are reacted to each other, a polymer is formed within a cell wall and the decorative veneer is solidified. Then the decorative veneer is dried, laminated to a base such as plywood, a ligneous material is manufactured and the ligneous material is used suitable as flooring.