Stabilized fluoropolymer and preparation thereof

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 -SO3M (in which M represents H, NR R R R or M 1/L ; R , R , R and R are the same or different and each represents H or an alkyl group containing 1 to 4 carbon atoms; and M represents an L-valent metal), and the treatment target substance has a moisture content of not higher than 500 ppm by mass.