PURPOSE: To reduce a time lag and to perform a detecting operation easily, by detecting the thickness of flocculated flock in a gravity-filtration process.

CONSTITUTION: A sludge to be treated is well mixed and agitated in a mixing tank 4 with a proper quantity of high-molecular flocculant corresponding to the concentration of the sludge at the time of operation, and the mixture is sent, after a flucculation treatment, to a gravity-filtration process 5 with the aid of a belt press, a wedge-pressing process 6, and a press-squeezing process 7 in sequence, and is finally discharged out of the system as a treated sludge. When the thickness of the flucculated floc is changed by the fluctuation of the concentration of a sludge to be treated, the changing quantity is detected by a thickness detector 1 of a flocculated floc and is sent to a controller 2, and the flow rate of a high molecular flocculant is adjusted by a flow rate controller 3 to feed a proper quantity of the flocculant to the mixing tank 4. Thus, when a sludge to be treated of a constant flow rate is treated, the dehydration treatment is performed in low cost and in excellent dehydrationrate without using any expensive sludgeconcentration meter.