PURPOSE: To form a surface radom layer in necessary thickness within a short time without deforming the same in the foam molding of a dielectric antenna element.

CONSTITUTION: A foamable material prepared by mixing a resin material, a foaming agent and a dielectric constant control agent is injected into a mold 10 and foamed to mold a dielectric lens 1. The cavities 11a, 11b of the mold 10 are composed of a metal material having good heat conductivity and held to 50-70°C for 80-100sec. By this constitution, a hard radom layer 3 is thinly formed on the surface of a foamed main body part 2.