Blood processing filter

A blood processing filter, which may be used for white blood cell removal, includes a flexible container having an inlet and an outlet for blood. A filter medium disposed between the inlet and the outlet, and a flow passage-securing member disposed between the filter medium and the outlet. The filter medium includes a filter layer X1 including a filter component unit A1, and a filter layer Y including a filter component unit B. The filter layer X1 is disposed between the filter layer Y and the flow passage-securing member. Ventilation resistances of the filter component unit A1 and unit B are 5.0 Pa·s/m2 or more and less than 9.0 Pa·s/m2 and 9.0 Pa·s/m2 or more, respectively. Ventilation resistances of the filter layer X1 and the filter medium are 4.0 kPa·s/m or more and 20.0 kPa·s/m or less and 55.0 kPa·s/m or more and less than 75.0 kPa·s/m, respectively.