PURPOSE: To obtain a lightweight material composition made of a resin material as the polymeric insulation material and improved in weather resistance and tracking resistance.

CONSTITUTION: This material composition is prepared by selecting an EPDM rubber as the polymeric material, adding aluminum hydroxide and carbon black as fillers to the rubber and molding a kneaded mixture of three components by an injection molding means. As the carbon black, furnace-type high-grade one having a specific surface area/particle diameter ratio of 17.3 and a volatile content of 9.0%, a specific surface area/particle diameter ratio of 17.3 and a volatile content of 2.0%, a specific surface area/particle diameter ratio of 17.3 and a volatile content of 10.0% or a specific surface area/particle diameter ratio of 11.7 and a volatile content of 3.5% is adopted.