The present invention is a method which is capable of producing crystals having a mean particle diameter larger than conventionally produced crystals, by carrying out a crystallizing method so-called neutralization crystallization. An ingredient organic acid salt solution is supplied as a reaction initial liquid in a reaction vessel (2). An acid is supplied via a dropping tube (10) to the ingredient organic acid salt solution, so that crystals of a targeted organic acid is precipitated. Then, the base is supplied in the reaction vessel (2) via a dropping tube (15), thereby partially dissolving the crystals being precipitated. The acid is further supplied from the dropping tube (10) for carrying out the crystallization.