PURPOSE: To obtain the treated water of superhigh purity by providing a 1st reverse osmosis membrane separator, a 2nd reverse osmosis membrane separator treating the water permeated through the 1st separator, and an electrodialysis unit consisting of alternatively formed concn. and dilution chambers.

CONSTITUTION: The device is provided with the 1st reverse osmosis membrane separator (the 1st RO) 1, the 2nd reverse osmosis, membrane separator (the 2nd RO) 2 treating the water permeated through the 1st RO, and the electrodialysis unit (UDI) 3. In this constitution, the raw water, such as city water, is firstly supplied to the 1st RO 1 to remove electrolytes and TOC components. Then, the water permeated through the 1st RO 1 is sent to the 2nd RO 2 where the electrolytes and the TOC components are removed again. In this case, the concentrated water in the 1st RO 1 is discharged through a piping 13 to the outside of the system, and the concentrated water in the 2nd RO 2 is circulated through a piping 15 to the upstream side of the 1st RO 1. The water permeated through the 2nd RO 2 is supplied to the concn. chamber and the dilution chamber through pipings 14a, 14b to be carried out the electrodialysis in the CDI 3. Thus, as the raw water is treated by two stages of RO, the treated water of superhigh purity is obtained.