This invention relates to a method for treating water by using a precoated filter. Since the method of the present invention utilizes ion-exchange fibers having an ion-exchange polymer and a reinforcing polymer as the precoating material together with a cation-exchange resin powder and an anion-exchange resin powder, the method made it possible to prominently prolong the lifetime of the precoating material and to improve the quality of the treated water. The method for treating water of the present invention is s~ited for the treatment of water to be used in, or waste eater from atomic power plants, steam power plants, pharmaceutical companies and the like.