A method for operating a filtration device (F1, F2, F3, F4, F5), wherein the filtration device (F1, F2, F3, F4, F5) comprises: a filter element having a filter medium constituted of a metal mesh material, in which during filtration to pass a fluid from one surface side to the other surface side of the filter medium, substances in the fluid are captured on the one surface, and during backwashing to pass a fluid from the other surface side to the one surface side of the filter medium, the captured substances are separated from the filter medium; a brush arranged to be capable of making a relative movement with respect to the one surface of the filter medium and assisting in separating the captured substances and sifting with respect to the captured substances during the relative movement; a driving unit relatively moving at least one of the brush or the filter element with respect to the other; a filtration mechanism (6, 7, 8, 9, V I, V O) passing a fluid from the one surface side to the other surface side of the filter medium and a backwashing mechanism passing a fluid from the other surface side to the one surface side of the filter medium; a casing supporting at least the filter element, the filtration mechanism (6, 7, 8, 9, V I, V O), and the backwashing mechanism; and a control unit controlling the driving unit, the filtration mechanism (6, 7, 8, 9, V I, V O), and the backwashing mechanism, and wherein the operating method of a filtration device (F1, F2, F3, F4, F5) comprises starting the backwashing when a differential pressure between the both surfaces of the filter medium exceeds a predetermined value or as triggered by a lapse of a predetermined time while performing the filtration, and returning to the filtration after the differential pressure is eliminated or after a predetermined time elapses, and the control unit performs a sifting operation of driving the driving unit to relatively move the brush with respect to the filter medium so as to sift removal unnecessary substances, that have particle diameters smaller than the openings and are not desired to be removed, from the captured substances and pass these removal unnecessary substances to the other surface side of the filter medium during the filtration. a filter element having a filter medium constituted of a metal mesh material, in which during filtration to pass a fluid from one surface side to the other surface side of the filter medium, substances in the fluid are captured on the one surface, and during backwashing to pass a fluid from the other surface side to the one surface side of the filter medium, the captured substances are separated from the filter medium; a brush arranged to be capable of making a relative movement with respect to the one surface of the filter medium and assisting in separating the captured substances and sifting with respect to the captured substances during the relative movement; a driving unit relatively moving at least one of the brush or the filter element with respect to the other; a filtration mechanism (6, 7, 8, 9, V I, V O) passing a fluid from the one surface side to the other surface side of the filter medium and a backwashing mechanism passing a fluid from the other surface side to the one surface side of the filter medium; a casing supporting at least the filter element, the filtration mechanism (6, 7, 8, 9, V I, V O), and the backwashing mechanism; and a control unit controlling the driving unit, the filtration mechanism (6, 7, 8, 9, V I, V O), and the backwashing mechanism, and wherein the operating method of a filtration device (F1, F2, F3, F4, F5) comprises starting the backwashing when a differential pressure between the both surfaces of the filter medium exceeds a predetermined value or as triggered by a lapse of a predetermined time while performing the filtration, and returning to the filtration after the differential pressure is eliminated or after a predetermined time elapses, and the control unit performs a sifting operation of driving the driving unit to relatively move the brush with respect to the filter medium so as to sift removal unnecessary substances, that have particle diameters smaller than the openings and are not desired to be removed, from the captured substances and pass these removal unnecessary substances to the other surface side of the filter medium during the filtration.