PURPOSE: To remarkably reduce the accumulation of sludge by providing a bottom plate in internal filtering device communicated with the passage holes of the inner partitioning plate in respective rooms and a passage space along the bottom plate between bottom surface of the filter member, thereby prohibiting proliferating configuration of bacteria to be as a filter film type.

CONSTITUTION: In the filter medium room (b) flowing-in water from passage hole 6 of inner partitioning plates 3 becomes a forced stream along the bottom plate 4. When the filter member layers are present thereon, bacterias are made to flow out by the dynamic pressure of the stream and lose its functions, thus, an adverse effect by stream is not caused by changing the stream into the bottom passage. The stream is abutted on the wall face 18 and stirred and buffered and made to flow to the passage hole 6' of inner partitioning plate 3, cause rising stream in space layers 8-8" by the pushing back condition and meanwhile penetrated and dispersed into the filter member 16 and cleaned by the contact with bacteria. In this exemplified case, excelsior type filter member having large passage space is used, so these actions are surely executed. Since the utilization of filter medium having large passage space, partial passage of water is prevented by the buffering action of the bottom passage so convenience utilizing only merits of the filtration medium is obtained.