PURPOSE: To obtain a stable and highly concentrated sludge, by controlling the height of an inlet opening of a sludge supplying pipe in accordance with the concentration and property of the sludge at an inlet.

CONSTITUTION: When a raw sludge is flowed into a fixed feed well 5 from the upper side, the slurry is formed into a sector-shaped gentle jet-flow from a delivery port 2 at the lower end of the well to flow toward the bottom 3 of a thickener 1, and a slurry of large specific gravity is concentrated to the lower part, and a clear supernatant water is separated upward. When a distance Ho between the bottom 3 and a movable feed well 7 is necessarily reduced because of the change of property of a raw slurry, a drum 15 is slowly rotated to rewind a rope 13 by driving a geared motor 14 equipped with a brake, and the movable feed well 7 moved downward by its own weight.