PURPOSE: To treat sludge below the water surface with good efficiency by a method in which a box is set to cover a sludge layer, a shaft tube is lowered while turning the shaft in the box, and cement slurry is discharged while agitating the sludge by a stirring blade.

CONSTITUTION: While turning a shaft 15 in a box 3 set underwater by covering a sludge layer 2 below the matter surface 1, the shaft tube 7 is lowered. The sludge layer 2 is drilled by the head 16 and excavated sludge is agitated by a stirring blade 17 and floated in water. Cement slurry is discharged from the holes 18 of the shaft 15 to form the cement slurry-sludge mixture floated in water. The floating of sludge on the water surface and the occurrence of offensive odors during the agitating operation can thus be prevented.