To provide a terminal group conversion method and a terminal stabilizing method, which are capable of converting unstable terminal groups in a fluorinated polymer to carboxylic acid fluoride groups at a high conversion rate. A fluorinated polymer is heated at from 200 to 400° C. for at least one hour in the presence of at least 50 mol of molecular oxygen per 1 mol of unstable terminal groups in the fluorinated polymer, to convert from 50 to 100 mol % of the unstable terminal groups to carboxylic acid fluoride groups.
