A blood processing filter comprises an inlet-side flexible container and an outlet-side flexible container that sandwich a filter element, an inlet port, and an outlet port, the filter further comprising: a flow path securing sheet a first seal portion and a second seal portion, wherein the flow path securing sheet includes a pair of ribs disposed opposite to each other with the outlet port interposed therebetween, and a slit formed inside of the pair of ribs, and diffusing openings that are disposed outside of the pair of ribs and continuously open from the ribs to the lateral portions of the first seal portion.
