To prevent a base material from absorbing moisture by applying a waterproof resin on the surface of the base material mainly made of rock wool, and sticking a decorated plywood to the upper face of the base material to mold an interior construction board.

A waterproof resin made of a thermosetting resin such as urethane or epoxy is applied, impregnated, and hardened on a base material 1 mainly made of rock wool, and a waterproof skin layer is formed on the surface of the base material 1. A male rabbet 4a and a female rabbet 4b are formed as rabbet sections at the outer peripheral end sections of the base material 1, a groove 3 is provided on the upper face of the base material 1, and a large quantity of the waterproof resin is applied to form a thick waterproof skin layer. A decorated veneer 2 is stuck on the upper face of the base material 1 by an adhesive, and surface coating is applied on the surface of the decorated veneer 2 with a sponge roll or a flow coater to mold an interior construction board 5. Waterproofness can be increased.