To provide a process for efficiently producing a perfluoropolymer having unstable terminal groups reduced. A process for producing a fluorination-treated perfluoropolymer, which comprises melting a thermoplastic perfluoropolymer to form strands and contacting the polymer strands with a fluorine gas. For fluorination-treatment of the perfluoropolymer, an apparatus 10 is used, which comprises a melt extruder 11 for melting and extruding the perfluoropolymer, a die 12 for forming the melt extruded polymer into continuous strands 1 and a fluorination tank 13 for contacting the continuous strands 1 with a fluorine gas.