PURPOSE: To facilitate speed control and maintenance of stirring mechanism, to make the structure simpler, and to reduce the installation cost by using a part of plural rows of stirring basin as pilot group and the other rows as follow-up group, and adjusting the speed of the stirring mechanism with a frequency changer.

CONSTITUTION: When change of turbidity, etc. occur in feed water, rotational frequency of stirring mechanism 38, 38 for the first stage stirring basin 5, 5 and the third stirring basin 7, 7 in the pilot group 30p is adjusted by operating pilot frequency changers 31a, 31b, and optimum stirring speed of the stirring mechanism 38 is set. And follow-up frequency changers 32a, 32b, 33a, 33b, 34a, 34b are operated following up the pilot frequency changer 31a, 31b to perform speed control of the stirring mechanism 38 of each stirring basin in the third follow-up group. Thus, the control of all stirring mechanisms can be performed easily and efficiently. Since the speed adjustment is performed with the frequency changer, the installation cost is reduced and the maintenance is facilitated.