A chlorosulfonated polyethylene polymer composition is disclosed, comprising a chlorosulfonated polyethylene polymer having incorporated therein (a) from 0.1 to 20 parts by weight of a metal salt of an organic acid, (b) from 0.1 to 10 parts by weight of a phenylamine and/or a phenylenediamine, (c) from 1 to 30 parts by weight of an acid acceptor, and (d) from 0.1 to 10 parts by weight of a vulcanizing agent, per 100 parts by weight of the chlorosulfonated polyethylene polymer. The composition provides a vulcanized product excellent in ozone resistance as well as mechanical strength.