PURPOSE: To reflect an electromagnetic wave as well coming from the outside by a special layer when a sealed space is formed by a building material and to make the building material to exhibit a shielding effect, by a structure wherein the layer which is mixed with a special conductive fiber or powder is formed on at least one side section of the building material.

CONSTITUTION: A board 1 in which a building material of, for example, a gypsum board is mixed with 50∼100% special conductive fiber or powder is provided. And, a layer 3 is provided which is formed by sticking a sheet of synthetic resin, woven fabric or non woven fabric which is mixed a special conductive fiber or powder, or by coating synthetic resin liquid which is mixed with a special conductive fiber or powder, on one side of a board 2 of a plank or synthetic resin. A shielding room 7 is formed by using the boards 1, 2 as a ceiling material, a floor material and a sidewall material, respectively.