PURPOSE: To keep the silica concn. in treated water low by forming I-type and II-type strongly basic anion-exchange resin beds respectively in the upper and lower chambers of an ion-exchange tower, exchanging ions by the upward flow and regenerating the resin by the downward flow.

CONSTITUTION: Liquid-permeable partition plates 3a-3c for supporting an ion- exchange resin bed are provided in the ion-exchange tower 1 to separate the tower into the upper and lower chambers 4 and 5. A I-type strongly basic anion- exchange resin bed 9 (e.g., trimethylammonium salt type) is formed in the upper chamber 4, and a II-type strongly basic anion-exchange resin bed 10 (e.g., methyl-2-hydroxyethylammonium salt type) is formed in the lower chamber 5. A liq. is passed upwardly through the lower chamber and the upper chamber to exchange ions, and the ion-exchange resin is regenerated by the downward flow from the upper chamber toward the lower chamber. As a result, raw water having a high content of silica is efficiently treated, the silica concn. in the treated water is kept low, and the resin is efficiently regenerated with a small amt. of regenerant.