A method for producing a fluoropolymer comprising polymerizing a fluoromonomer in an aqueous medium in the presence of the surfactant defined below to provide a fluoropolymer, wherein the surfactant comprises at least one of the compounds of the formulae (a) and (b): R 1a -C(=O)-R 2a -C(=O)-R 3a -OSO 3 X a (a) wherein R 1a is linear or branched C≥ 1 -alkyl or C≥ 3 -cycloalkyl, each optionally substituted by OH or a monovalent organic group which contains an ester bond and optionally contains a carbonyl group when containing ≥ 2 carbon atoms and optionally contains a monovalent or divalent heterocycle or optionally forms a ring when containing ≥ 3 carbon atoms; R 2a is a single bond or a divalent linking group; R 3a is a single bond or a divalent linking group; R 1a -R 3a contain ≥ 6 carbon atoms in total; and any two of R 1a -R 3a optionally bind to each other to form a ring; and X a is H, a metal atom, imidazolium optionally containing a substituent, pyridinium optionally containing a substituent, phosphonium optionally containing a substituent, or NR 4a 4 wherein R 4a each independently is H or an organic group; R 1b -C(=O)-(CR 2b 2) n - (OR 3b) p -(CR 4b 2) q -L-OSO 3 X b (b) wherein R 1b is unsubstituted linear or branched C 1-3 -alkyl; R 2b each individually is H or unsubstituted linear or branched C 1-3 -alkyl; n is an integer of ≥ 1; R 3b is optionally substituted C 1-10 -alkylene; q is an integer of ≥ 0; R 4b each individually is H or unsubstituted linear or branched C 1-3 -alkyl; p is an integer of ≥ 0; and any two of R 1b -R 4b optionally bind to each other to form a ring; and L is a single bond, -CO 2 -B-*, -OCO-B-*, -CONR 6b -B-*, -NR 6b CO-B-*, or -CO-other than the carbonyl groups in -CO 2 -B-, -OCO-B-, -CONR 6b -B-, and -NR 6 CO-B-, where B is a single bond or optionally substituted C 1-10 -alkylene, R 6b is H or optionally substituted C 1-4 -alkyl, and * indicates the bond to -OSO 3 X b; and X b is H, a metal atom, imidazolium optionally containing a substituent, pyridinium optionally containing a substituent, phosphonium optionally containing a substituent, or NR 5b 4 wherein R 5b each independently is H or an organic group. wherein the surfactant comprises at least one of the compounds of the formulae (a) and (b): R 1a -C(=O)-R 2a -C(=O)-R 3a -OSO 3 X a (a) wherein R 1a is linear or branched C≥ 1 -alkyl or C≥ 3 -cycloalkyl, each optionally substituted by OH or a monovalent organic group which contains an ester bond and optionally contains a carbonyl group when containing ≥ 2 carbon atoms and optionally contains a monovalent or divalent heterocycle or optionally forms a ring when containing ≥ 3 carbon atoms; R 2a is a single bond or a divalent linking group; R 3a is a single bond or a divalent linking group; R 1a -R 3a contain ≥ 6 carbon atoms in total; and any two of R 1a -R 3a optionally bind to each other to form a ring; and R 2a is a single bond or a divalent linking group; R 3a is a single bond or a divalent linking group; X a is H, a metal atom, imidazolium optionally containing a substituent, pyridinium optionally containing a substituent, phosphonium optionally containing a substituent, or NR 4a 4 wherein R 4a each independently is H or an organic group; R 1b -C(=O)-(CR 2b 2) n - (OR 3b) p -(CR 4b 2) q -L-OSO 3 X b (b) wherein R 1b is unsubstituted linear or branched C 1-3 -alkyl; R 2b each individually is H or unsubstituted linear or branched C 1-3 -alkyl; n is an integer of ≥ 1; R 3b is optionally substituted C 1-10 -alkylene; q is an integer of ≥ 0; R 4b each individually is H or unsubstituted linear or branched C 1-3 -alkyl; p is an integer of ≥ 0; and any two of R 1b -R 4b optionally bind to each other to form a ring; and R 1b is unsubstituted linear or branched C 1-3 -alkyl; R 2b each individually is H or unsubstituted linear or branched C 1-3 -alkyl; n is an integer of ≥ 1; R 3b is optionally substituted C 1-10 -alkylene; q is an integer of ≥ 0; R 4b each individually is H or unsubstituted linear or branched C 1-3 -alkyl; p is an integer of ≥ 0; L is a single bond, -CO 2 -B-*, -OCO-B-*, -CONR 6b -B-*, -NR 6b CO-B-*, or -CO-other than the carbonyl groups in -CO 2 -B-, -OCO-B-, -CONR 6b -B-, and -NR 6 CO-B-, where B is a single bond or optionally substituted C 1-10 -alkylene, R 6b is H or optionally substituted C 1-4 -alkyl, and * indicates the bond to -OSO 3 X b; and X b is H, a metal atom, imidazolium optionally containing a substituent, pyridinium optionally containing a substituent, phosphonium optionally containing a substituent, or NR 5b 4 wherein R 5b each independently is H or an organic group.