To provide a refining method for the high efficiency of desalting treatment, particularly to provide a method for desalting a water-soluble dye used in ink-jet printing ink and a miniaturized apparatus.

This method for refining a water-soluble organic compound by using a reverse osmosis membrane involves a pretreatment comprising adding water to the compound so as not to completely dissolve therein into a slurry which is then put to filtration. The refining apparatus comprises a vessel 1 having at least two external circulating paths equipped with pumps 3, 4 and piping 9, 10.