PURPOSE: To provide the quality of an outlet liquid so as to be within a specified range by preventing the variation of water of outlet liquid following the variation of condition of feed liquid.

CONSTITUTION: The quality of outlet liquid from a centrifugal cleaner 1 is measured, and the number of rotations of a motor 2 is controlled so as to provide the concentration of an aimed substance comes into a specified range. A device of finding the number of rotations of the motor 2 from the outlet liquid flow rate and the aimed substance concentration is disposed on a concentration measuring instrument 7 for measuring the quality of outlet liquid, and the number of rotations of the motor 2 is controlled on line. In the case the feed liquid flow rate and the feed liquid composition are varied, the variation is sensed by the concentration measuring instrument 7 and the number of rotations of the motor 2 is controlled to set the quality of outlet liquid from a clean liquid line 9 within the specified range.