There is provided a method for producing a C2-C20 fluorine-containing organic acid with reduced fluorine-free organic compound. A method for producing a C2-C20 fluorine-containing organic acid, the method including contacting a mixture containing a C2-C20 fluorine-containing organic acid and a fluorine-free organic compound with a concentrated sulfuric acid and then separating a C2-C20 fluorine-containing organic acid phase from a concentrated sulfuric acid phase. The contacting is conducted such that an amount of water present in the concentrated sulfuric acid phase is 10% by mass or less, and the C2-C20 fluorine-containing organic acid is obtained in the form of the C2-C20 fluorinated organic acid phase having a reduced content ratio of the fluorine-free organic compound compared with said mixture.