A free organic acid can be produced efficiently at a low cost, by subjecting an ammonium salt of organic acid A such as a dicarboxylic acid, a tricarboxylic acid or an amino acid, to reactive crystallization with an acid B, such as a monocarboxylic acid, wherein organic acid A and acid B satisfy the following formula (1): pKa(A).ltoreq.pKa(B) (1) where pKa(A) and pKa(B) represent ionization indices of organic acid A and acid B, respectively, provided that when they have plural values, they represent the minimum pKa among them, to separate free organic acid A in solid form.