PURPOSE: To prevent generation of obstruction for electric waves by spreading a netting material over the surface of a heat insulating plate, spraying mortar and carbon fiber separately so that the netting material is embedded, and thereby forming a substratum.

CONSTITUTION: A meshing material 25 consisting of glass mesh, luth, etc., is spread over the surface of a heat insulating material 1 such as extruded foaming polystylene, foaming urethane, etc. From a mortar nozzle 11, cement mortar 13 is jetted so that the meshing material 25 is embedded, while carbon fiber 14 is spouted out of a nozzle therefore 12, and the two are mixed and deposited on the heat insulating material 1. After deposition of the admixture, the surface is graded with a roller 15 so that the surface 3a of substratum is made smooth. This ensures less secular deterioration and unlikelihood of the generation of cracks.