PURPOSE: To obtain radio wave absorber which has suitable complex dielectric contact and can be used adequately in the outdoors, by forming foam composed of specific foamable silicon having parts wherein the laterally sectional area increases gradually in the travelling direction of radio waves, in order to obtain the matching with the space.

CONSTITUTION: Radio wave absorber 22 is constituted of a metal plate 16 like aluminum and foam 20 whose foaming magnification is 1.5-6.0. When the foaming magnification is 1.5 or smaller, the reduction effect of specific gravity is small, so that the superiority to solid state silicon is not obtained and radio wave absorbing performance is decreased. As compared with the state before foaming, the volume is not so increased, so that molding by a mold is difficult. When the forming magnification is 6 or larger, the mechanical strength is decreased, and the material is not suitable for radio wave absorber. In order to optimize the radio wave absorption capability, carbon fibers having a length of 0.5-10.0mm are dispersed by the density of 0.0001-0.02g/cm3. The shape of the foam 20 is constituted as a pyramid type like a circular cone or a wedge type.