PURPOSE: To obtain the cake having an optimum water content by changing the supply of sludge, the feed rate of chemicals, and the differential velocity of a centrifugal dehydrator by using the turbidity as an intermediate variable.

CONSTITUTION: A turbidimeter 13 for detecting the turbidity in a centrifugal dehydrator 4 is provided in addition to an existing device. In addition an arithmetic unit 12 for the supply of sludge and the feed rate of chemicals is provided to take in the turbidity and the supply of sludge detected by a sludge supply meter 3, to output the set value of the supply of sludge and the feed rate of chemicals respectively to PID controllers 10 and 11, and also to output a differential velocity value to a dehydrator differential velocity command unit 14 for controlling a centrifugal dehydrator driving motor 15. The supply of sludge, the feed rate of chemicals, and the differential velocity of the centrifugal dehydrator are changed by using the turbidity as an intermediate variable, and the cake having an optimum water content can be obtained.