PURPOSE: To easily and highly separate and purify a water-soluble substance in a short time by impressing an equifield on an ion-exchange membrane or a material having a unidirectional array of polar groups.

CONSTITUTION: Water as solvent is pumped up by a metering pump 2 from a tank 1 to send water in fixed portions to a column cell 3. A DC or bipolar pulse voltage is impressed on two parallel metallic electrode plates 4 in the column cell 3 from a bipolar high-voltage pulse power source 5 to generate an equifield in the region between the parallel electrode plates 4 including an ion-exchange membrane 6. A substance to be separated and purified is injected from an injection port 7, and the substance is adsorbed on the ion-exchange membrane 6 impressed with the equifield. The equifield is then reduced to zero, and impurities are washed away. Subsequently, the substance is desorbed from the ion-exchange membrane 6 by changing the strength of the electric field in a continuous gradient or a discontinuous gradient.