PURPOSE: To reduce injection quantities of coagulants by injecting an inorganic coagulant into a raw water in accordance with a changed injection factor of the inorganic coagulant and a measured flow rate value of the raw water and by injecting an organic coagulant into the raw water in accordance with the injection factor variation value of the organic coagulant and the measured flow rate value of the raw water.

CONSTITUTION: A chemical feeding to a coagulator is carried out in the following processes. a) An injection factor variation of an inorganic coagulant is determined by a fuzzy logic reasoning based on a measured turbidity value of a treated water and a calculated turbidity variation value of the treated water. b) The injection factor variation of an organic high polymer coagulant is determined by the fuzzy logic reasoning based on the measured turbidity value of the treated water and the calculated turbidity variation value of the treated water. c) A selected injection factor value of the inorganic coagulant is changed in accordance with the assumed injection factor value of the inorganic coagulant, which is determined in process a), and the inorganic coagulant is injected into a raw water in accordance with a changed injection factor value of the inorganic coagulant and a measured flow rate value of the raw water. d) The selected injection factor value of the high polymer organic coagulant is changed in accordance with the assumed injection factor variation value of the organic high polymer coagulant, and this coagulant is injected in accordance with the injection factor variation value of the organic high polymer coagulant and the measured flow rate value of the raw water.