A device for concentrating an object to be treated has a filter body in which a movable plate is disposed between adjacent annular fixed plates, the moveable plate being formed in the same annular shape. Sludge filtrate passes through filtrate inflow gaps between the fixed plates and the movable plate before being caused to flow into the filter body. The movable plate is moved so that solids in the sludge are kept from clogging the filtrate inflow gaps. A cleaning member is caused to rub against the outer peripheral surfaces of the fixed plates so that solids adhering to the outer peripheral surfaces of the fixed plates are scraped off. The movable plates of said device are caused to perform a circular movement by eccentric cams fixed to the shaft. The cleaning member is fixed to the shaft, and caused to rotate around the shaft by the rotation of the shaft so that solids adhering to the outer peripheral surfaces of the fixed plates are scraped off.