A modified fluorine-containing copolymer obtained by irradiating a copolymer with radiation at a temperature of not higher than the melting point of the copolymer. The copolymer includes at least one copolymer selected from a copolymer including a tetrafluoroethylene unit and a perfluoro(alkyl vinyl ether) unit and a copolymer including a tetrafluoroethylene unit and a hexafluoropropylene unit, and has 10 to 10000 functional groups in total per 106 carbon atoms.