PURPOSE: To provide a flocculating and filtration tank for water treatment by which treated water of the desired water quality is stably obtained without the need of controlling the injected quantity of a focculant by an operator because of the injected quantity of the focculant into raw water being automatically controlled.

CONSTITUTION: A pH signal SP and a turbidity signal ST representing pH and turbidity of treated water 16 respectively are supplied to a controller 38. A control loop which is previously determined so that the injected quantity of a flocculant 26 suitable for pH and turbidity of the treated water 16 may be determined by the controller 39 including a membership function having pH and turbidity of the treated water 16 as parameter is operated based on fuzzy inference to determine the injected quantity of the flocculant 26 and simultaneously to operate a flow control valve 28 so that the injected quantity thus determined may be attained.