A method for producing an -SO 3 H group-containing fluoropolymer comprising terminal stabilization treatment of a fluoropolymer (FP) containing - A monomer unit containing a group -SO 2 X wherein X is F or Cl, and - an unstable terminal group which is any one selected from groups of the formulae - : -CFT-R 5 (OH) n1 -CFT-(R 6 ) n2 -OR 7 -CFT-(R 8 ) n3 -COR 9 -CFT-(R 10 ) n4 -OCOOR 11 -CFT-CONR 12 R 13 -CFT-COOR 14 -CFT-R 15 wherein T is F, or C 1-10 -perfluoroalkyl or C 2-15 -perfluoroalkoxy each optionally containing one or more ether oxygen atoms and/or -SO 2 X groups, R 5 -R 15 each independently are a C 1-10 -hydrocarbon group optionally partially substituted by halogen, and R 12 , R 13 and R 15 each independently may further be H; n1 is an integer of 1-3, and n2-n4 each independently are 0 or 1; which method comprises the steps of (P) treating the fluoropolymer (FP) by (i) Contacting it with an oxidizing agent and then hydrolyzing the thus-formed -CF 2 COF groups; or (ii) treating it at 200-300°C and then hydrolyzing the resulting -CF 2 COF groups; or (iii) reacting it (A) with a halogenating agent, and then (B) with a liquid or gaseous decomposition treatment agent comprising water, a fluoroalcohol or a mixture thereof; to obtain an -SO 3 H group-containing fluoropolymer in which at least 90% of the unstable terminal groups are -CFTCO 2 Z, wherein T is as defined above, and Z is H, NR 1 R 2 R 3 R 4 or an alkali metal element, and R 1 -R 4 each independently are H or C 1-4 -alkyl; and (Q) converting -CFTCO 2 Z to -CF 2 T by reacting with a fluorinating agent at ≥ 0°C to < 150°C. - A monomer unit containing a group -SO 2 X wherein X is F or Cl, and - an unstable terminal group which is any one selected from groups of the formulae - : -CFT-R 5 (OH) n1 -CFT-(R 6 ) n2 -OR 7 -CFT-(R 8 ) n3 -COR 9 -CFT-(R 10 ) n4 -OCOOR 11 -CFT-CONR 12 R 13 -CFT-COOR 14 -CFT-R 15 wherein T is F, or C 1-10 -perfluoroalkyl or C 2-15 -perfluoroalkoxy each optionally containing one or more ether oxygen atoms and/or -SO 2 X groups, R 5 -R 15 each independently are a C 1-10 -hydrocarbon group optionally partially substituted by halogen, and R 12 , R 13 and R 15 each independently may further be H; n1 is an integer of 1-3, and n2-n4 each independently are 0 or 1; -CFT-R 5 (OH) n1 -CFT-(R 6 ) n2 -OR 7 -CFT-(R 8 ) n3 -COR 9 -CFT-(R 10 ) n4 -OCOOR 11 -CFT-CONR 12 R 13 -CFT-COOR 14 -CFT-R 15 T is F, or C 1-10 -perfluoroalkyl or C 2-15 -perfluoroalkoxy each optionally containing one or more ether oxygen atoms and/or -SO 2 X groups, R 5 -R 15 each independently are a C 1-10 -hydrocarbon group optionally partially substituted by halogen, and R 12 , R 13 and R 15 each independently may further be H; n1 is an integer of 1-3, and n2-n4 each independently are 0 or 1; (P) treating the fluoropolymer (FP) by (i) Contacting it with an oxidizing agent and then hydrolyzing the thus-formed -CF 2 COF groups; or (ii) treating it at 200-300°C and then hydrolyzing the resulting -CF 2 COF groups; or (iii) reacting it (A) with a halogenating agent, and then (B) with a liquid or gaseous decomposition treatment agent comprising water, a fluoroalcohol or a mixture thereof; to obtain an -SO 3 H group-containing fluoropolymer in which at least 90% of the unstable terminal groups are -CFTCO 2 Z, wherein T is as defined above, and Z is H, NR 1 R 2 R 3 R 4 or an alkali metal element, and R 1 -R 4 each independently are H or C 1-4 -alkyl; and (i) Contacting it with an oxidizing agent and then hydrolyzing the thus-formed -CF 2 COF groups; or (ii) treating it at 200-300°C and then hydrolyzing the resulting -CF 2 COF groups; or (iii) reacting it (A) with a halogenating agent, and then (B) with a liquid or gaseous decomposition treatment agent comprising water, a fluoroalcohol or a mixture thereof; (Q) converting -CFTCO 2 Z to -CF 2 T by reacting with a fluorinating agent at ≥ 0°C to < 150°C. <EOS>