The present invention provides a method for producing a stabilized fluoropolymer which comprises producing the stabilized fluoropolymer by subjecting a treatment target substance containing a sulfonic-acid-derived-group-containing fluoropolymer to a fluorination treatment, wherein the sulfonic-acid-derived-group-containing fluoropolymer is a fluoropolymer containing -SO 3 M (in which M representsH, NR 1 R 2 R 3 R 4 or M 1 1/L ; R 1 , R 2 , R 3 and R 4 are the same or different and each represents H or an alkyl group containing 1 to 4 carbon atoms; and M 1 represents an L-valent metal), and the treatment target substance has a moisture content of not higher than 500 ppm by mass.