A rubber composition includes, as essential components, an EDPM polymer, a carbon black, an electrical insulating amorphous or spindle-shaped filler, a softener, and a sulfur-based vulcanizing agent as essential components, and also includes, as an optical component, an electrical insulating plate-shaped filler. An obtained vulcanized product is satisfied with the following Equations (1) and (2): volume resistivity ≧1.0×106 Ω·cm  (1) (M30 /M30 −1)×100≧−15%  (2) where M30 is a tensile stress at elongation of 30% as measured by a tensile test at 23° C., and M30 is a tensile stress at elongation of 30% as measured by a tensile test at 100° C.