Various purification methods are applicable to the magnesium salt, which results in a favorable form of this salt during the fermentation process. Furthermore, and in view of the methods disclosed in JP2001-514900A and JP2004-196768A which do not generate salt by-products, a method in which the salt is substituted was proposed to convert the ammonium salt into a more favorable form (JP2005-295998A). The magnesium compound produced by substituting the salt is reused as a neutralizing agent in the fermentation process. Magnesium succinate which is produced in the fermentation process is dissolved in the culture solution, and usually does not precipitate as crystals. Therefore, the salt substitution step is carried out by adding ammonium carbonate or the like to the culture solution. As a result, the mixture in the culture solution, other than magnesium succinate, is carried into the ammonium succinate solution which is the starting material for the purification process. Therefore, an additional step is necessary to remove the complicated mixture before or after the salt substitution step.