When sulfinating a carboxylic acid bromofluoroalkyl ester by using a sulfinating agent, an organic base is used, thereby obtaining a fluoroalkanesulfinic acid ammonium salt. This is oxidized to obtain a fluoroalkanesulfonic acid ammonium salt. This is used as the raw material and is converted to an onium salt or is converted to an onium salt by going through saponification and esterification, thereby obtaining a fluoroalkanesulfonic acid onium salt. This fluoroalkanesulfonic acid onium salt is useful as a photoacid generator used for chemically amplified resist materials, etc.