This invention relates to a blood processing filter including a sheet-like filter element, and a flexible container having an internal space that is divided into one side and another side by the filter element. The flexible container includes: a first container forming part and a second container forming part that are arranged opposite each other so as to sandwich the filter element therebetween ports that are provided in the first container forming part and the second container forming part, respectively, and that serve as an inlet or an outlet for blood a band-shaped annular first seal part that seals the first container forming part and the filter element, and that is formed so as to surround the port provided in the first container forming part and an annular second seal part that seals at least the first container forming part and the second container forming part, and that is formed so as to surround the first seal part along an outer edge of the second container forming part. The first container forming part includes an interior filtration space forming part and an exterior surrounding space forming part that are separated by the first seal part. The second container forming part includes an extension space forming part that is surrounded by the second seal part, and is arranged opposite the filter element and the surrounding space forming part, without being divided by the first seal part. The port of the first container forming part is provided in the filtration space forming part and communicates with an internal space on the one side that is 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 the other side that is defined by the filter element, and protrudes further than an outer surface of the extension space forming part.
