NOVELTY - A monomer mixture comprising vinylidene fluoride and hexafluoropropylene is added to reactor, and copolymerization reaction is carried out in presence of ammonium-2,3,3,3-tetrafluoro-2-(1,1,2,3,3,3-hexafluoro-2-(1,1,2,2,3,3,3-heptafluoropropoxy)propoxy)-1-propanoate. 30-70 wt.% of monomer mixture is added to primary phase and copolymerization reaction is carried out in presence of chain transfer agent. 0.3-1.5 wt.% chain transfer agent is added to secondary phase of reaction material, and copolymerization reaction is carried out, to obtain fluorine-containing elastomer.    USE - Manufacture of fluorine-containing elastomer used for fuel system component e.g. fuel hose and tube for fuels e.g. methanol, gasoline, light oil and naphtha.    ADVANTAGE - The method provides fluorine-containing elastomer having excellent physical property, workability, adhesiveness, heat resistance, solvent resistance, chemical resistance, and compression-set resistance.    DETAILED DESCRIPTION - A monomer mixture comprising vinylidene fluoride and hexafluoropropylene is added to reactor, and copolymerization reaction is carried out in presence of ammonium-2,3,3,3-tetrafluoro-2-(1,1,2,3,3,3-hexafluoro-2-(1,1,2,2,3,3,3-heptafluoropropoxy)propoxy)-1-propanoate. 30-70 wt.% of monomer mixture is separately-added to primary phase with respect to total amount of monomer mixture and copolymerization reaction is carried out in presence of chain transfer agent. 0.3-1.5 wt.% chain transfer agent is added to secondary phase of reaction material, and copolymerization reaction is carried out, to obtain fluorine-containing elastomer. The content of chain transfer agent is 0.0001-0.1 wt.% with respect to total amount of reaction material. The fluorine-containing elastomer comprises copolymerization composition including 10-40 mol% tetrafluoroethylene, 30-80 mol% vinylidene fluoride and 10-30 mol% hexafluoropropylene. The content of tetrafluoroethylene with respect to total amount of monomer mixture is 10-40 wt.%.