PURPOSE: To efficiently clean the eutrophication pollution in a closed type water area by dispersing the deposits of bottom material forcibly into the water so that the polluting components are taken by living matters.

CONSTITUTION: An aerator 2 and/or mechanical stirrer 3 is installed to a movable raft-like structural body 1 and a double pipe type contact device having inside cylinders 4, 4' and outside cylinders 5, 5' is provided toward the inside of the water. The bottom material deposits are forcibly dispersed and returned by such aerator 2 or stirrer 3 into the water to additionally dig out the deposits as needed. While the deposits are fluidized together with the living matters in the bottom material and water and are circulated by the double pipe type contact device, a high degree of contact is attained and the polluting components such as org. materials, phosphorus-contg. materials, nitrogen-contg. materials and metal salts in the deposits are taken by the living matters as the nutrition sources. The eutrophication pollution in the closed type water area is thus effectively cleaned.