A blood separation method using a blood separation filter (10A to 10F) that separates a predetermined blood component from blood, the blood separation filter (10A to 10F) including: a housing (18, 62); a filter medium disposed in the housing (18, 62) and partitioning the housing (18, 62) into a blood inflow chamber and a blood outflow chamber in a thickness direction of the housing (18, 62); an inflow port provided in the housing (18, 62) to allow blood to flow into the blood inflow chamber; and an outflow port provided in the housing (18, 62) to allow post-separation blood from which a blood component has been separated by the filter medium to flow out from the blood outflow chamber, the blood separation method comprising: an arrangement step of arranging the housing (18, 62) such that the blood inflow chamber is positioned vertically below the filter medium and the blood outflow chamber is positioned vertically above the filter medium; a blood treatment step of first allowing the blood to flow from the inflow port into the blood inflow chamber in a state of the arrangement step, allowing the blood to flow through the inside of the filter medium upward from vertically below, and then allowing the post-separation blood inside the blood outflow chamber to flow out to the outflow port; and a post residual treatment blood collection step of arranging the housing (18, 62) such that the outflow port is positioned vertically below the blood outflow chamber after the blood treatment step, thereby guiding post-separation residual blood in the housing (18, 62) to the outflow port. a housing (18, 62); a filter medium disposed in the housing (18, 62) and partitioning the housing (18, 62) into a blood inflow chamber and a blood outflow chamber in a thickness direction of the housing (18, 62); an inflow port provided in the housing (18, 62) to allow blood to flow into the blood inflow chamber; and an outflow port provided in the housing (18, 62) to allow post-separation blood from which a blood component has been separated by the filter medium to flow out from the blood outflow chamber, the blood separation method comprising: an arrangement step of arranging the housing (18, 62) such that the blood inflow chamber is positioned vertically below the filter medium and the blood outflow chamber is positioned vertically above the filter medium; a blood treatment step of first allowing the blood to flow from the inflow port into the blood inflow chamber in a state of the arrangement step, allowing the blood to flow through the inside of the filter medium upward from vertically below, and then allowing the post-separation blood inside the blood outflow chamber to flow out to the outflow port; and a post residual treatment blood collection step of arranging the housing (18, 62) such that the outflow port is positioned vertically below the blood outflow chamber after the blood treatment step, thereby guiding post-separation residual blood in the housing (18, 62) to the outflow port. an arrangement step of arranging the housing (18, 62) such that the blood inflow chamber is positioned vertically below the filter medium and the blood outflow chamber is positioned vertically above the filter medium; a blood treatment step of first allowing the blood to flow from the inflow port into the blood inflow chamber in a state of the arrangement step, allowing the blood to flow through the inside of the filter medium upward from vertically below, and then allowing the post-separation blood inside the blood outflow chamber to flow out to the outflow port; and a post residual treatment blood collection step of arranging the housing (18, 62) such that the outflow port is positioned vertically below the blood outflow chamber after the blood treatment step, thereby guiding post-separation residual blood in the housing (18, 62) to the outflow port.