A nitrile group-containing copolymer rubber contains α,β-ethylenically unsaturated nitrile monomer units in a ratio of 10 to 60 wt % and has a Z-average radius of gyration of 100 nm or more. Preferably, the nitrile group-containing copolymer rubber has an iodine value of 120 or less. Further, preferably, the nitrile group-containing copolymer rubber has a plasticity number of 14 to 90. It is possible to provide a nitrile group-containing copolymer rubber able to give a cross-linked rubber excellent in tensile stress, compression set resistance, and low heat buildup property.