Composition production method, and composition

A method for producing a composition, the method comprising a step A of performing ultrafiltration, microfiltration, dialysis membrane treatment, or a combination thereof on a composition comprising water and a water-soluble fluoropolymer, wherein the fluoropolymer is a polymer comprising a structural unit M3 derived from a monomer represented by general formula (1):

• CX2═CY(—CZ2—O—Rf-A) (1)
• wherein
• X is the same or different and is —H or —F;
• Y is —H, —F, an alkyl group, or a fluorine-containing alkyl group;
• Z is the same or different and is —H, —F, an alkyl group, or a fluoroalkyl group;
• Rf is a fluorine-containing alkylene group having 1 to 40 carbon atoms or a fluorine-containing alkylene group having 2 to 100 carbon atoms and having an ether bond;
• and
• A is —COOM, —SO3M, —OSO3M, or —C(CF3)2OM, wherein M is —H, a metal atom, —NR74, imidazolium optionally having a substituent, pyridinium optionally having a substituent, or phosphonium optionally having a substituent, and R7 is H or an organic group;
• provided that at least one of X, Y, and Z comprises a fluorine atom,
• wherein a content of the structural unit M3 in the fluoropolymer is 60 mol % or more based on all polymerization units.