A fluorine-containing copolymer obtained by copolymerizing tetrafluoroethylene, hexafluoropropylene and perfluoro vinyl ether as component monomers, wherein a weight ratio of tetrafluoroethylene, hexafluoropropylene and perfluoro vinyl ether units constituting the fluorine-containing copolymer is 70 to 95: 5 to 20: 0 to 10, respectively; the fluorine-containing copolymer having: a melt flow rate of 30 (g/10 minutes) or more; a volatile content index of 0.2% by weight or less; and a stress relaxation modulus G(t) (unit: dyn/cm2) which satisfies the following formula at t=0.1 second when measured at a temperature of 310° C.: G(0.1)>7×106×X−1.62−3000 where X denotes the melt flow rate (unit: g/10 minutes). Also disclosed is an insulating material composed of the fluorine-containing copolymer and an insulated cable having a core conductor coated with the fluorine-containing copolymer.