Provided are a rubber-based composition modifier that gives a vulcanizate superior in heat resistance and ozone resistance and a vulcanizate and a molded product of a rubber composition containing the modifier. The rubber-based composition modifier comprises 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 m 2 /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; and the acetylene black has a nitrogen adsorption specific surface area (N 2 SA), as determined by the method specified in JIS K6217-2, of 50 to 160 m 2 /g.