A membrane filter unit includes a hollow fiber membrane module with a plurality of sheet-like hollow fiber membrane elements arrayed in parallel at predetermined intervals, each of the hollow fiber membrane elements obtained by arranging a great number of porous hollow fiber membranes in parallel, and an air diffuser arranged below the hollow fiber membrane module for releasing minute bubbles toward a lower end of the hollow fiber membrane module and generating a gas-liquid mixed flow vertically swirling between an inner space and an outer space of the hollow fiber membrane module. The membrane filter unit has a residue eliminating mechanism for forming a forced flow in part of the mixed flow between the porous hollow fiber membranes and between the sheet-like hollow fiber membrane elements of the fiber membrane module.