PURPOSE: To prevent corrosion from occurring to floor boards or groundsills by a method wherein charred bamboo boards formed by pressure molding of a material made by mixing charred bamboo grains or carbonized bamboo pellets with cement are laid under a floor and used as an underfloor moisture adjustment material.

CONSTITUTION: Carbonized charred bamboo is formed by applying dry distillation to the bamboo at the temperature of 400 to 700°C, and charred bamboo grains in average grain size of 2 to 7mm are made by crushing. Then, a charred bamboo board 1 is formed by pressure molding of an admixture made by mixing the charred bamboo grains or carbonized bamboo pellets or carbonized bamboo fibers in 60 to 90wt.% and resin binder in 3 to 7wt.% with cement. The charred bamboo boards 1 are laid over the ground 3 under the floor 2 with their joints connected with adhesive agent 4, and thereby underfloor moisture adjustment becomes possible. Therefore, excellent moisture adjustment and gas absorptivity can be achieved easily.