To provide a high-efficiency membrane-filtration apparatus, wherein SS concentration in contact with the membrane surface of a membrane filtration apparatus can be reduced, and a high flux operation can be performed, the installation scale after a sludge-dehydration operation can be minimized, and removal of phosphor and nitrogen can also be conducted.

This apparatus includes an aerobic tank 2, and a membrane separation tank 4 provided with membrane separation equipment 5. To-be-treated water is allowed to flow into the aerobic tank 2 for aerobic treatment. In the membrane separation equipment 5, aerobically treated water from the tank 2 is separated into water subjected to the membrane filtration and sludge. The membrane filtration apparatus includes a sludge concentrator 9 for concentration of the sludge. The membrane filtration apparatus includes further a denitrification tank 15 for removing nitrogen, the aerobic tank 2 for aerobic treatment, and the membrane separation tank 4 provided with the membrane separation equipment 5, and further the sludge concentrator 9 for concentration of sludge. Further, a coagulant-addition tank 14 is provided to the aerobic tank 2.