This invention provides a blood processing filter including a flexible container having an inlet and an outlet for blood, and a sheet-like filter media arranged so as to divide the inside of the flexible container into one side and another side. An annular seal part is formed at which an entire circumference that is adjacent to a peripheral portion of the filter media is integrally sealed to the flexible container. The blood processing filter is retained in a curved shape that curves as viewed from a predetermined one direction that is orthogonal to the thickness direction by the rigidity of the seal part.
