A nitrile group-containing copolymer rubber comprising 10 to 40 wt % of α,β-ethylenically unsaturated nitrile unit (a), 10 to 60 wt % of α,β-ethylenically unsaturated carboxylic acid ester unit (b), and 20 to 70 wt % in total of conjugated diene unit (c) and saturated conjugated diene unit (d), wherein a content ratio “x” of a conjugated diene unit (c) based on the total content of said conjugated diene unit (c) and said saturated conjugated diene unit (d) is 0.08 or smaller, and “x·A” defined as a product of said ratio “x” and a content ratio “A” (%) of said α,β-ethylenically unsaturated nitrile unit (a) is 1.5 or smaller. According to the present invention, a nitrile group-containing copolymer rubber capable to provide a rubber vulcanizate highly balanced in mechanical strength, heat aging resistance, oil resistance, cold resistance and ozone resistance, and a vulcanizable rubber composition using the rubber can be provided.