PURPOSE: To obtain an ultrapure water wherein electrolytes, fine particles, org. substances, etc., are thoroughly removed and to make maintenance of regenerating agent, etc., unnecessary by connecting an electrophoresis unit using a monovalent cation selectively permeable membrane and a reverse osmosis unit in this order.

CONSTITUTION: Raw water is at first treated in an electrophoresis unit 1 and then, is stored in a treated water tank 2 and is transferred to a reverse osmosis unit 4, wherein it is treated and the obtd. ultrapure water is sent to a ultra-pure water storing tank 5. On the other hand, the conc. water discharged from the reverse osmosis unit 4 is sent to a concn. chamber of the electrophoresis unit of the front stage and is used as a conc. liq. In addition, as a monovalent cation selectively permeable membrane is used in the electrophoresis unit of the front stage, monovalent cations in the raw water are selectively moved to obtain a treated liq. contg. cations being at least divalent. Then, the cations being at least divalent are removed at the reverse osmosis unit of the latter stage.