To provide a novel method that permits efficient manufacture at a low cost of a free organic acid having a high melting point from an ammonium salt of the organic acid obtained by microbial conversion of a carbon resource in the presence of a neutralizing agent while recycling and reusing reaction raw materials used or by-products without disposing of them.

An ammonium salt of an organic acid A such as a dicarboxylic acid, a tricarboxylic acid or an amino acid is subjected to reaction crystallization, using an acid B such as a monocarboxylic acid satisfying equation (1) below to separate the free organic acid A as a solid. pKa(A)pKa(B) (1) (provided that pKa(A) and pKa(B) are the respective electrolytic dissociation constants of the organic acid A and the acid B and, when they have a plurality of electrolytic dissociation constants, represent the minimum pKa among them).