PURPOSE: To prevent electromagnetic waves from leaking inside or outside, by a method wherein metallic foil, which has a specified thickness and of which the extended part has a specified size, is stuck on either surface of a flame-retaradant expanded polystyren base-board with a specified thickness, and an inorganic board with a specified thickness is stuck thereon.

CONSTITUTION: Metallic foil 3 is stuck on either of the upper and lower surfaces a 10-100mm thick base-board 1 flame-retardant expanded polystyrene beads molding or of an extruded foam polystyrene molding. In this case, the metallic foil 3 is made at a thickness of 20 to 200μm, the metallic foil 3 is extended by 1 to 100mm at adjacent sides of the base board 1, and parts to be joined with adjacent members are formed. After that, an inorganic board 5 having a thickness of 9.5 to 25mm is stuck on the metallic foil 3. At this time, an adhesive 2 with which the metallic foil 3 is stuck on the base board 1 and with which the inorganic board 5 is stuck on the metallic foil 3, is made of urethane resin, silicone resin or epoxy resin. In the case where an enclosed space is formed of this building material, electromagnetic waves generated at the inside of the space is prevented from leaking outside. Furthermore, this building material is effective as a shield against the electromagnetic waves.