PURPOSE: To concentrate sludge with small labor by storing a filter bag with coarse meshes in a filter bag container with a filtered water-outlet below, and controlling the transmission flow velocity of filtered water from the filter bag by a pressure action to the internal circumferential surface of the container when a mixture liquid of sludge is stored.

CONSTITUTION: A filter bag with coarse meshes which comes into contact under pressure with a slight gap (g) left with the internal circumferential surface and the internal bottom surface of the container 1 by a water pressure of a mixture liquid of sludge 6 stored is accommodated in a detachable manner within the filter bag container 1 with a filtered water discharge outlet 2 below. Then the flow velocity of filtered water from the filter bag 5 into the slight gap (g) is controlled by the afore-mentioned pressure action to the internal circumferential surface and the internal bottom surface of the container 1 when the mixture liquid of sludge is stored. As a result, a higher cost for equipment required conventionally for a large-size device is not required, and also the filter bag need not be cleaned due to clogging. Sludge can be concentrated with less labor and maintenance cost by extracting and treating sludge from a processing tank intermittently and at an unfixed timing.