A filtration system (100, 200) having a plurality of filtration tanks (130, 230) for filtering water to be treated, characterized in that the filtration system comprises: a first filtration tank having a first floating filter media layer formed of floating filter media, a first upper screen with a first aperture ratio (A 1), the first upper screen being disposed over the first floating filter media layer and supporting the floating filter media, a first inlet for said water to be treated disposed below the first floating filter media layer, a first backwash water supply source located over the first upper screen, and a first backwash water outlet means disposed below the first floating filter media layer and discharging backwash water supplied from the first backwash water supply source during backwashing of the first floating filter media layer; and a second filtration tank having a second floating filter media layer formed of floating filter media, a second upper screen with a second aperture ratio (A 2), the second upper screen being disposed over the second floating filter media layer and supporting the floating filter media, a second inlet for said water to be treated disposed below the second floating filter media layer, an inflow blocking mechanism capable of blocking inflow of water to be treated through the second inlet for water to be treated, a second backwash water supply source located over the second upper screen, and a second backwash water outlet means disposed below the second floating filter media layer and discharging backwash water supplied from the second backwash water supply source during backwashing of the second floating filter media layer, wherein the first aperture ratio (A 1) is smaller than the second aperture ratio (A 2), wherein the aperture ratio = (aperture area / upper screen area) X 100. a first filtration tank having a first floating filter media layer formed of floating filter media, a first upper screen with a first aperture ratio (A 1), the first upper screen being disposed over the first floating filter media layer and supporting the floating filter media, a first inlet for said water to be treated disposed below the first floating filter media layer, a first backwash water supply source located over the first upper screen, and a first backwash water outlet means disposed below the first floating filter media layer and discharging backwash water supplied from the first backwash water supply source during backwashing of the first floating filter media layer; and a second filtration tank having a second floating filter media layer formed of floating filter media, a second upper screen with a second aperture ratio (A 2), the second upper screen being disposed over the second floating filter media layer and supporting the floating filter media, a second inlet for said water to be treated disposed below the second floating filter media layer, an inflow blocking mechanism capable of blocking inflow of water to be treated through the second inlet for water to be treated, a second backwash water supply source located over the second upper screen, and a second backwash water outlet means disposed below the second floating filter media layer and discharging backwash water supplied from the second backwash water supply source during backwashing of the second floating filter media layer, wherein the first aperture ratio (A 1) is smaller than the second aperture ratio (A 2),