PURPOSE: To produce excellent aggregating effect, by measuring the alkalinity of raw water and the pH of treated water, then by controlling respective charging ratios by comparing the measured values with respective target values.

CONSTITUTION: Corrected alkalinity is obtained by an arithmetic unit 10 from the alkalinity of raw water measured by alkalinity meter 5 and the charging ratio of chlorine 7. The corrected alkalinity is compared with the predetermined target value 12 of corrected alkalinity by a comparator 11 to find the difference. Then the pH of treated water measured by a pH meter 6 is compared with the target pH value 14 of treated water by a comparator 13 to find the difference. By a comparator 15, alkali charging ratio and flocculant charging ratio are corrected from the difference between the pH of the treated water and the target pH of the treated water, and the difference between the corrected alkalinity and the corrected target value for alkalinity in consideration of the increase in the alkalinity of raw water due to the charge of alkali agent 8, and the decrease in the alkalinity due to the charge of flocculant 9. Hereby the pH of the treated water is kept constant at all time, hence excellent aggregating effect is obtained.