This invention relates to a blood processing filter including a band-shaped first seal part that seals an inlet-side flexible container of the blood processing filter and a filter element of the blood processing filter so as to surround an inlet port, and an annular second seal part that seals at least the inlet-side flexible container and an outlet-side flexible container and is formed surrounding the first seal part at a position that is closer to an outer edge than the first seal part. A valley part corresponding to the first seal part is provided on an outlet side of the filter element. An outlet opening that communicates with an inside of the outlet-side flexible container is formed in the outlet port. At least one portion of the outlet opening is arranged so as to overlap with the valley part.