BLOOD TREATMENT FILTER

An object is to provide a blood processing filter that is excellent in all of the white blood cell removal performance, filtration time (filtration rate), and the blood recovery. The object can be solved by the following blood processing filter.A blood processing filter comprising a flexible container having an inlet and an outlet for blood, a filter medium disposed between the inlet and the outlet of the flexible container, and a flow passage-securing member disposed between the filter medium and the outlet of the flexible container, wherein the filter medium comprises a filter layer X1 including a filter component unit A1, and a filter layer Y including a filter component unit B, wherein the filter layer X1 is disposed between the filter layer Y and the flow passage-securing member,a ventilation resistance per thickness of the filter component unit A1 is 5.0 Pa·s/m2 or more and less than 9.0 Pa·s/m2,a ventilation resistance per thickness of the filter component unit B is 9.0 Pa·s/m2 or more,a ventilation resistance of the filter layer X1 is 4.0 kPa·s/m or more and 20.0 kPa·s/m or less, anda ventilation resistance of the filter medium is 55.0 kPa·s/m or more and less than 75.0 kPa·s/m.