A rubber-based composition modifier includes 100 parts by mass of a mixed rubber component containing a chloroprene rubber and at least one copolymer selected from ethylene/α-olefin copolymers and ethylene/α-olefin/unconjugated polyene copolymers, 10 to 60 parts by mass of magnesium oxide, 3 to 30 parts by mass of a softener, and 30 to 120 parts by mass of acetylene black, wherein the mixed rubber component has a Mooney viscosity, as determined by the method specified in JIS K6300, of 60 to 180 at 100° C.; the magnesium oxide has a BET specific surface area, as determined by the one point method specified in JIS Z8830, of 20 to 150 m2/g; the softener contains a fatty acid ester-based plasticizer and an unsaturated fatty acid having a carbon number of 10 to 24 and containing an ester bond, a hydroxyl group, or both of them in part of the structure.