A method for processing a blood sample is provided that can improve the recovery rate of deformable rare cells, such as circulating tumor cells (CTCs), that would easily pass through a filter and small rare cells while reducing the filtration area of the filter, and that can recover the rare cells alive.In one aspect, the present disclosure relates to a method for separating or detecting rare cells in a blood sample using a filter (4) including holes with a hole density of at least 200 holes/mm2 to 40000 holes/mm2, wherein the holes are in the form of an ellipse with a minor axis diameter of 3.0 µm to 15 µm and a major axis diameter of 1.1 times to 3 times as long as the minor axis diameter, or the holes have a shape that contains the ellipse and is in contact with the ellipse at at least two points including both ends of the major axis of the ellipse. The method includes separating or detecting rare cells by filtering the blood sample in a volume of 6 µl/hole or less with respect to a hole of the filter (4).
