This invention relates to a blood processing filter including a flexible container having an inlet and an outlet for blood, and a sheet-like filter media that is assembled in the flexible container. The blood processing filter has a seal part that seals the flexible container and the filter media to form an effective filtration portion of the filter media and a partition part that seals the flexible container and the filter media to partition the effective filtration portion into a plurality of areas. The seal part and the partition part cooperate with the filter media to divide inside of the flexible container into three or more internal spaces including an inlet space that communicates with the inlet, and an outlet space that communicates with the outlet, and also form, as a blood channel, a channel that passes through each of three or more of the internal spaces and also passes multiple times through the filter media.
