This invention relates to a blood processing filter including a sheet-like filter element and a flexible container having an internal space divided by the filter element. The container includes first and second container-forming-parts. The first container-forming-part includes an interior filtration space-forming-part and an exterior surrounding space-forming-part that are separated by a first seal-part. The second container-forming-part includes an extension space-forming-part surrounded by the second seal-part and arranged opposite the filter element and the surrounding space-forming-part, without being divided by the first seal-part. A port of the first container-forming-part is provided in the filtration space-forming-part and communicates with an internal space on one side defined by the filter element. The port of the second container-forming-part is provided in the extension space-forming-part and communicates with an internal space on another side defined by the filter element, and protrudes further than an outer surface of the extension space-forming-part.