A blood processing filter (1A, 1B, 1C, 1D) comprising: a sheet-shaped filter element; an inlet-side flexible container and an outlet-side flexible container that sandwich the filter element; an inlet port that is provided at the inlet-side flexible container and receives unprocessed blood; and an outlet port that is provided at the outlet-side flexible container and discharges the blood processed by the filter element, the filter (1A, 1B, 1C, 1D) further comprising: a flow path securing sheet disposed between the filter element and the outlet-side flexible container; a belt-shaped first seal portion that seals at least the filter element and the flow path securing sheet, and is provided so as to be apart from the outlet-side flexible container; and an annular second seal portion that seals at least the inlet-side flexible container and the outlet-side flexible container, and is provided so as to surround the filter element and the flow path securing sheet, characterized in that the first seal portion includes a pair of lateral portions disposed opposite to each other with the outlet port interposed therebetween, and a communication portion (13a, 13b) connected to the pair of lateral portions, the communication portion (13a, 13b) includes a lower side portion and an upper side portion, the flow path securing sheet is a sheet which includes ribs consisting of only a pair of ribs that are disposed between the pair of lateral portions and opposite to each other with the outlet port interposed therebetween, a slit disposed between the pair of ribs to communicate with a concave portion formed of the communication portion (13a, 13b), and diffusing openings that are disposed outside of the pair of ribs, and continuously open from the ribs to the lateral portions and communicate with the concave portion formed of the lateral portions, and continuously open from the lower side portion to the upper side portion, and the width of each of the diffusing openings is 20 to 30 mm. a sheet-shaped filter element; an inlet-side flexible container and an outlet-side flexible container that sandwich the filter element; an inlet port that is provided at the inlet-side flexible container and receives unprocessed blood; and an outlet port that is provided at the outlet-side flexible container and discharges the blood processed by the filter element, the filter (1A, 1B, 1C, 1D) further comprising: a flow path securing sheet disposed between the filter element and the outlet-side flexible container; a belt-shaped first seal portion that seals at least the filter element and the flow path securing sheet, and is provided so as to be apart from the outlet-side flexible container; and an annular second seal portion that seals at least the inlet-side flexible container and the outlet-side flexible container, and is provided so as to surround the filter element and the flow path securing sheet, characterized in that the first seal portion includes a pair of lateral portions disposed opposite to each other with the outlet port interposed therebetween, and a communication portion (13a, 13b) connected to the pair of lateral portions, the communication portion (13a, 13b) includes a lower side portion and an upper side portion, the flow path securing sheet is a sheet which includes ribs consisting of only a pair of ribs that are disposed between the pair of lateral portions and opposite to each other with the outlet port interposed therebetween, a slit disposed between the pair of ribs to communicate with a concave portion formed of the communication portion (13a, 13b), and diffusing openings that are disposed outside of the pair of ribs, and continuously open from the ribs to the lateral portions and communicate with the concave portion formed of the lateral portions, and continuously open from the lower side portion to the upper side portion, and the width of each of the diffusing openings is 20 to 30 mm. a flow path securing sheet disposed between the filter element and the outlet-side flexible container; a belt-shaped first seal portion that seals at least the filter element and the flow path securing sheet, and is provided so as to be apart from the outlet-side flexible container; and an annular second seal portion that seals at least the inlet-side flexible container and the outlet-side flexible container, and is provided so as to surround the filter element and the flow path securing sheet, characterized in that the first seal portion includes a pair of lateral portions disposed opposite to each other with the outlet port interposed therebetween, and a communication portion (13a, 13b) connected to the pair of lateral portions, the communication portion (13a, 13b) includes a lower side portion and an upper side portion, the flow path securing sheet is a sheet which includes ribs consisting of only a pair of ribs that are disposed between the pair of lateral portions and opposite to each other with the outlet port interposed therebetween, a slit disposed between the pair of ribs to communicate with a concave portion formed of the communication portion (13a, 13b), and diffusing openings that are disposed outside of the pair of ribs, and continuously open from the ribs to the lateral portions and communicate with the concave portion formed of the lateral portions, and continuously open from the lower side portion to the upper side portion, and the width of each of the diffusing openings is 20 to 30 mm.