A blood separation method using a blood separation filter (10A) includes: an arrangement step of arranging a housing (18) such that a blood inflow chamber (20) is positioned below a filter member (24) and the blood outflow chamber (22) is positioned above a filter medium (38)； a blood treatment step of allowing blood to flow in the filter medium (38) upward from vertically below； and a post residual treatment blood collection step of arranging the housing (18) such that an outflow port (28) is positioned vertically below the blood outflow chamber (22) so as to guide the post-separation residual blood in the housing (18) to the outflow port (28).
