Provided are a high dielectric insulating silicone rubber composition and an electric field relaxation layer. This high dielectric and insulating silicone rubber comprises: (A) 100 parts by mass of an organopolysiloxane represented by formula (1), R1nSiO(4−n)/2  (1), (in the formula, R1 represents the same or different, substituted or unsubstituted monovalent hydrocarbon groups, and n is a positive number of 1.95 to 2.04); (B) 60 to 150 parts by mass of thermal black having an average primary particle size of 150 to 500 nm; (C) 5 to 100 parts by mass of reinforcing fumed silica having a specific surface area measured by a BET adsorption method of 50 m2/g or more; and (D) 0.1 to 10 parts by mass of a curing agent. The composition is characterized in that a cured product thereof satisfies a dielectric constant of 10 or more, a volume resistivity of 1.0×1013 to 1.0×1017 Ω·cm, a loss tangent of 0.1 or less, and a dielectric breakdown strength (BDV) of 7 kV/mm or more.