PURPOSE: To decrease the moisture contents of dehydrated sludge and to reduce the costs of sludge treatments by providing moisture content meters which measure the moisture contents of the dehydrated sludge discharged into dehydrators, and controlling the amt. of the sludge to be supplied to the dehydrators according to the measured values thereof.

CONSTITUTION: When a start instruction is applied to pump 3 to draw the sludge, a sludge control device 24 starts a dehydrator 11, and when a start instruction is applied to a pump 4, said device starts a dehydrator 12. When a solenoid vave 7 opens, sludge is supplied to a dehydrator 10. The moisture content of the dehydrated sludge which is dehydrated in the dehydrator 10 is measured with a moisture content meter 21, and is applied to the device 24. When the actually measured value of moisture content is higher than a prescribed value, the device 24 operates the valve 7 in the direction to close to decrease the amt. of the sludge to be supplied into the dehydrator 10. When the actually measured value of moisture content is lower than the prescribed value, said device operates the valve 7 in the direction to open to increase the amt. of the sludge to be supplied.