To control the flocculation quantity of phosphoric acid ions by controlling the elution quantity of iron ions becoming a flocculant by simple operation.

First and second electrodes 204, 206 made of iron are arranged in a treatment tank 202 and respectively connected to two power supplies 210, 220 through conductors 208 and switches 212, 222. The interval between the first and second electrodes 204, 206 is set to a range of 1-100cm. At a time of operation, a current is supplied within a range of potential difference of 0.1-100V, pref., 0.5-20V and, by operating the switches 212, 222, the direction of a current is changed to a reverse direction at every definite time to alternately change an anode and a cathode. By this operation, the elution quantity of iron ions can be kept constant.