A high-permittivity rubber compound (MX) to be over 10, 15, or 20 in relative permittivity is produced by filling, to 100 weight parts of base material (M10), 300 or more, 400 or more, or 500 or more weight parts of barium titanate family material as powder (M11) over 2000 in relative permittivity at temperatures from room temperature to 90.degree. C. A high-permittivity rubber compound (M30, M60) crosslinked by peroxide is heat-treated (S21, S31) to remove decomposition residue of the peroxide, and powder (BT4) of barium titanate family material of industrial grade is washed (S09) by de-ionized water (Mc) to remove ionic impurities, suppressing degradation of insulating characteristics.
