PURPOSE: To reduce a ratio wave trouble area by confining reflected radio waves around a building by mounting cylindrical metal grids on a plane wall surface of a building and permitting the grids to manifest the same radio wave reflection capability as that of the cylindrical metal plate.

CONSTITUTION: A metal grid A includes a longitudinal grid material 1 and a transversal grid material 2 both comprising light metal such as aluminum. The metal grid A is cylindrically mounted on the outside of a plane wall surface of a building in a direction of a radio field of the incoming radio wave matching the size of the grids to the wavelength of the radio wave. Hereby, reflected radio waves are confined around the building to reduce a radio wave trouble area.