A membrane filtration device comprising a plurality of membrane separation units being arranged in vertically spaced stages at different heights, wherein each of the membrane separation units has a primary side, to which supplied raw water is received, the primary side communicating with a backwash drainage system and a flushing-air supply system, and a secondary side from which permeate is collected, the secondary side communicating with a backwash-water supply system, the backwash-water supply system includes a backwash-water main pipe disposed vertically along the plurality of membrane separation units in the vertically spaced stages for passing backwash water and a plurality of horizontal backwash-water branch pipes connected in parallel to the backwash-water main pipe so as to feed the backwash water into the membrane separation units in the vertically spaced stages, the backwash water being supplied downward or upward into the backwash-water main pipe and fed into the membrane separation units in the vertically spaced stages through the backwash-water branch pipes, the backwash drainage system includes a plurality of horizontal backwash-drainage branch pipes into which backwash drain water flows from the membrane separation units in the vertically spaced stages, and a vertical backwash-drainage main pipe for passing backwash drain water downward, the backwash-drainage branch pipes being connected to the backwash-drainage main pipe in parallel, and the flushing-air supply system includes an upwardly leading air main pipe disposed vertically along the plurality of membrane separation units in the vertically spaced stages for passing flushing air upward, an air supply unit connected to a lower end of the air main pipe, and a plurality of horizontal air branch pipes for feeding flushing air into the membrane separation units in the vertically spaced stages, the air branch pipes being connected in parallel to the air main pipe, wherein each of the air branch pipes leads to a different stage of said vertically spaced stages, the flushing air flowing upward in the air main pipe filled with raw water supplied during membrane filtration, and the flushing air being fed into the membrane separation units in the vertically spaced stages through the air branch pipes. wherein each of the membrane separation units has a primary side, to which supplied raw water is received, the primary side communicating with a backwash drainage system and a flushing-air supply system, and a secondary side from which permeate is collected, the secondary side communicating with a backwash-water supply system, the backwash-water supply system includes a backwash-water main pipe disposed vertically along the plurality of membrane separation units in the vertically spaced stages for passing backwash water and a plurality of horizontal backwash-water branch pipes connected in parallel to the backwash-water main pipe so as to feed the backwash water into the membrane separation units in the vertically spaced stages, the backwash water being supplied downward or upward into the backwash-water main pipe and fed into the membrane separation units in the vertically spaced stages through the backwash-water branch pipes, the backwash drainage system includes a plurality of horizontal backwash-drainage branch pipes into which backwash drain water flows from the membrane separation units in the vertically spaced stages, and a vertical backwash-drainage main pipe for passing backwash drain water downward, the backwash-drainage branch pipes being connected to the backwash-drainage main pipe in parallel, and the flushing-air supply system includes an upwardly leading air main pipe disposed vertically along the plurality of membrane separation units in the vertically spaced stages for passing flushing air upward, an air supply unit connected to a lower end of the air main pipe, and a plurality of horizontal air branch pipes for feeding flushing air into the membrane separation units in the vertically spaced stages, the air branch pipes being connected in parallel to the air main pipe, wherein each of the air branch pipes leads to a different stage of said vertically spaced stages, the flushing air flowing upward in the air main pipe filled with raw water supplied during membrane filtration, and the flushing air being fed into the membrane separation units in the vertically spaced stages through the air branch pipes.