To provide an electromagnetic wave attenuation control method for satisfactorily performing radio communication by appropriately controlling attenuation quantity of electromagnetic wave intensity.

Radio LAN equipment 12A and 12B is arranged in a living room 12, and transmission/reception are performed by an electromagnetic wave for radio LAN between them. A building material 1802 attenuating an interfering electromagnetic wave to a degree (about -20 dB to -30 dB) that transmission/ reception are not interfered by the electromagnetic wave for radio LAN, which are performed between radio LAN equipment 12A and 12B, is disposed in a part through which the interfering electromagnetic wave W passes in an outer wall 18.