A differential rate rotary thickener; a power detector which is disposed in a sludge receiver tank for thickened sludge thickened in the differential rate rotary thickener; a discriminator which receives an electric signal for a thickened sludge concentration detected by the power detector and which calculates and discriminate the data; a first controller which receives an instruction signal being a discriminated result in the discriminator and which operates an outer cylinder driving machine and a screw driving machine; a ratio setter which receives a discriminated signal of the discriminator and which increases or decreases a chemical feed rate of flocculant stepwise; and a second controller which receives an instruction signal from the ratio setter and which operates a flocculant-feeding pump are provided. Thickening of sludge is performed by controlling the chemical feed rate, a rotational speed of a screw, and a rotational speed of an outer cylinder screen.