A blood treatment filter for removing undesirable components, such as aggregates and leukocytes, from blood is provided. The filter comprises a filter sheet (5) between sealed flexible inlet- and outlet-side containers (9, 11), an inlet port (2), a tubular outlet port (4), an outlet-side frame sheet (7) between the filter (5) and the outlet-side container (11)； a belt-shaped first seal (13) provided by sealing the filter and the outlet-side frame sheet (7)； and an annular second seal (15) provided by sealing the inlet-side flexible container (9), the frame sheet (7), the outlet port (4) and the outlet-side flexible container (11) in this order to surround the first seal. On the outlet-side of the filter (5), the first seal (13) forms a valley with a bottom (6a) and slopes (6b, 6c). The outlet has an opening (41) overlapping with the first seal (13).
