A high-hardness rubber composition which is obtained by vulcanizing an unvulcanized rubber composition comprisingan unvulcanized ethylene..alpha.-olefin rubber composition (A) comprising 100 parts by weight of an ethylene..alpha.-olefin copolymer rubber, 60-150 parts by weight of a reinforcing agent and 0-70 parts by weight of a softener;5-40 parts by weight of a solid diene rubber (B) based on 100 parts by weight of said ethylene..alpha.-olefin copolymer rubber; and4-15 parts by weight of sulfur (C) based on 100 parts by weight of said ethylene..alpha.-olefin copolymer rubber,wherein said unvulcanized rubber composition is obtained by adding said solid diene rubber (B) and said sulfur (C) to said unvulcanized ethylene..alpha.-olefin rubber composition (A).