To obtain high quality treated water of 10MΩ.cm or more only by an RO membrane separation apparatus.

RO membrane separation apparatuses 3, 4, 5 are arranged over three stages and a means adding alkali 7 to the water supplied to the first RO membrane separation apparatus 3 to adjust pH and a means adding alkali 8 to the water supplied to the second RO membrane separation apparatus 4 to adjust pH are provided. In this case, by adding alkali to the water supplied to the first and second RO membrane separation apparatuses 3, 4 to adjust pH over two stages to perform three-stage RO treatment, treated water of markedly high quality can be obtained. An ion exchange resin required heretofore can be removed and the simplification of the maintenance and control of the apparatus, the enhancement of treatment efficiency and the reduction of treatment cost can be achieved.