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 EPDM rubber, and molding the kneaded mixture of these three components by an injection molding means. As the carbon black, furnace-type general-purpose and high-grade one having a specific surface area/particle diameter ratio of 3.3 and a volatile content of 0.6%, a specific surface area/particle diameter ratio of 6.0 and a volatile content of 0.8%, a specific surface area/ particle diameter ratio of 6.8 and a volatile content of 0.8%, a specific surface area/particle diameter ratio of 4.2 and a volatile content of 0.8%, a specific surface area/particle diameter ratio of 5.7 and a volatile content of 1.1% or a specific surface area/particle diameter ratio of 4.0 and a volatile content of 0.3% is adopted.