A blood processing filter including a flexible container (having an inlet and an outlet and a sheet-like filter member that is assembled in the flexible container) has a seal part that seals the flexible container and the filter member to form an effective filtration portion of the filter member and a partition part that seals the flexible container and the filter member to partition the effective filtration portion into a plurality of areas. The seal part and the partition part cooperate with the filter member 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 member.
