PURPOSE: To provide a board material suitable as a ceiling material, a lathing material or a wall covering material.

CONSTITUTION: An aqueous dispersion of a carbonyl group-containing copolymer containing a hydrazine derivative is added to a substrate composed of an inorganic substance such as kaolinite and the resultant mixture is then formed into the required shape of a board. Thereby, the objective thin and lightweight flexible board rich in rubber-like elasticity, capable of exhibiting firm binding action and excellent in impact resistance, dimensional stability, etc., is obtained and execution can extremely readily be carried out.