PURPOSE: To separate an aq. soln. to a desalted liquid contg. impurities at a lower ratio and a concd. liquid of a small volume.

CONSTITUTION: Plural stages of electrodialysis cells 100 provided with desalting chambers 107 and concentrating chambers 106 by cation exchange membranes 103 and anion exchange membranes 104 are provided. The aq. soln. is successively passed through the desalting chambers 107 of such plural electrodialysis cells 100 and the concd. liquid supplied into the concentrating chambers 106 of the plural electrodialysis cells 100 is successively passed in the direction reverse from the aq. soln., by which the aq. soln. is separated to the desalted liquid decreased in the materials exclusive of the water and the concd. liquid concd. with the materials exclusive of the water.