A novel method is provided whereby a free organic acid can be produced particularly from an ammonium salt of an organic acid having a high melting point obtainable by bioconversion of a carbon source in the presence of a neutralizing agent, efficiently at a low cost, and the used material for reaction and a byproduct can be recycled for reuse without being disposed. An ammonium salt of organic acid A such as a dicarboxylic acid, a tricarboxylic acid or an amino acid is subjected to reactive crystallization by means of acid B such as a monocarboxylic acid satisfying the following formula (1), to separate free organic acid A in solid form: 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. The crystallization mother liquor after precipitating and separating organic acid A is, after separating acid B and then an ammonium salt of acid B, recycled for use in the reactive crystallization step. The ammonium salt of acid B is decomposed into acid B and ammonia, which are recycled for use in the reactive crystallization step and as a neutralizing agent in the bioconversion step, respectively.