This invention relates to a blood processing filter including 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 provided in the outlet-side flexible container for discharging blood after being processed by the filter element. The blood processing filter also includes 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 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. 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.
