Organic acid ammonium solution is produced by the steps of obtaining a fermentation broth containing organic acid magnesium by using an organic acid-producing microorganism in the presence of a magnesium compound, producing organic acid ammonium and a magnesium compound by subjecting the organic acid magnesium contained in the fermentation broth to salt-exchange using an ammonia compound, and separating the produced magnesium compound to obtain organic acid ammonium solution.