A blood processing filter (1A, 1B) comprising a sheet-like filter element, an inlet-side flexible container and an outlet-side flexible container that sandwich the filter element and are sealed thereto, an inlet port provided in the inlet-side flexible container for accepting blood before being processed by the filter element, and an outlet port (11a, 11g) provided in the outlet-side flexible container for discharging blood after being processed by the filter element; the blood processing filter (1A, 1B) further comprising: a band-shaped first seal part that seals the inlet-side flexible container and the filter element so as to surround the inlet port; and an annular second seal part that seals at least the inlet-side flexible container and the outlet-side flexible container, and that is formed so as to surround the first seal part at a position that is closer to an outer edge than the first seal part; wherein: the outlet-side flexible container is not bonded to the filter element; a valley part corresponding to the first seal part is provided on an outlet side of the filter element; the valley part includes a bottom part that overlaps with the band-shaped first seal part, an inner slanted face portion that rises towards the inside of the band-shaped first seal part from the bottom part, and an outer slanted face portion that rises towards the outside of the band-shaped first seal part; an outlet opening (11c, 11h) that communicates with an inside of the outlet-side flexible container is formed in the outlet port (11a, 11g); and at least one portion of the outlet opening (11c, 11h) is arranged so as to overlap with the valley part. a band-shaped first seal part that seals the inlet-side flexible container and the filter element so as to surround the inlet port; and an annular second seal part that seals at least the inlet-side flexible container and the outlet-side flexible container, and that is formed so as to surround the first seal part at a position that is closer to an outer edge than the first seal part; wherein: the outlet-side flexible container is not bonded to the filter element; a valley part corresponding to the first seal part is provided on an outlet side of the filter element; the valley part includes a bottom part that overlaps with the band-shaped first seal part, an inner slanted face portion that rises towards the inside of the band-shaped first seal part from the bottom part, and an outer slanted face portion that rises towards the outside of the band-shaped first seal part; an outlet opening (11c, 11h) that communicates with an inside of the outlet-side flexible container is formed in the outlet port (11a, 11g); and at least one portion of the outlet opening (11c, 11h) is arranged so as to overlap with the valley part. the outlet-side flexible container is not bonded to the filter element; a valley part corresponding to the first seal part is provided on an outlet side of the filter element; the valley part includes a bottom part that overlaps with the band-shaped first seal part, an inner slanted face portion that rises towards the inside of the band-shaped first seal part from the bottom part, and an outer slanted face portion that rises towards the outside of the band-shaped first seal part; an outlet opening (11c, 11h) that communicates with an inside of the outlet-side flexible container is formed in the outlet port (11a, 11g); and at least one portion of the outlet opening (11c, 11h) is arranged so as to overlap with the valley part.