The purpose of the present invention is to obtain an ethylene.α-olefin.non-conjugated polyene copolymer that has a low permanent compression set at low temperatures, is flexible, and has an excellent balance of rubber elasticity at low temperatures and tensile strength at normal temperatures. This ethylene-based polymer is an ethylene.α-olefin.non-conjugated polyene copolymer that includes units derived from ethylene (A), units derived from an α-olefin (B) containing 4-20 carbon atoms, and units derived from a non-conjugated polyene (C) and satisfies (1)-(4). (1) The molar ratio of (A) to (B) is 40/60-90/10, (2) the contained amount of the units derived from (C) is 0.1-6.0 mol %, (3) ML(1+4)125° C. is 5-100, and (4) the B value is 1.20 or more.