A fluorine-containing alloyed copolymer, which comprises a fluorine-containing elastomer copolymer having a fluorine content of not less than 64% by weight, and a resinous tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer having an amount of heat of crystal fusion ΔH of not more than 10 J/g, can give crosslinked moldings having a distinguished plasma resistance against both CF4 plasma treatment and O2 plasma treatment, and also a distinguished cracking resistance at the plasma irradiation, and showing necessary normal state physical properties and compression set when used as seals, and capable of providing soft seals of low hardness, and attaining molding at relatively low temperatures.