PURPOSE: To remove impurities from liquid effectively by feeding powdery ion exchange resin or precoating additive continuously or intermittently in the stage of passing the liquid to be treated.

CONSTITUTION: A mixed precoated layer (initial stage precoated layer) 2a consisting of powdery ion exchange resin and precoating additive is formed in the first stage on a filter element 1. Then, the liquid to be treated is fed to the initial stage precoated layer 2a and is passed therethrough. In this stage, the powdery ion exchange resin and/or the precoated additive 5 is(are) supplied continuously or intermittently to form a fresh precoated layer (secondary precoated layer) 2b successively on the initial stage precoated layer 2a. As the result, the surface layer of the precoated layer is renewed. Accordingly, the filtration is not executed by the cake filtration mode but is executed by the standard clogging filtration mode, so the increase of pressure drop is retarded. Therefore, the durability of operation is prolonged, further, suspended materials particularly, colloidal materials, in the initial treating liquid are removed effectively.