A nitrile group-containing highly saturated copolymer rubber composition includes a nitrile group-containing highly saturated copolymer rubber (A) having α,β-ethylenically unsaturated nitrile monomer units and (meth)acrylic acid ester monomer units and having an iodine value of 120 or less and a polyether ester-based plasticizer (B) having structural unit made of three or more ether compounds which are successively bonded. A nitrile group-containing highly saturated copolymer rubber composition excellent in vulcanization speed and giving cross-linked rubber has excellent heat resistance and cold resistance.