Blood is separated into blood components aseptically in a closed system by first filtering whole blood from a collector (1) through a filter (2) for removing leukocytes or removing leukocytes and platelets, to produce filtered blood, which is passed to and collected in a primary bag (3) fluid-tightly connected to the filter (2) through a sealable, cuttable conduit (9), and the conduit (9) is sealed and cut to disconnect the primary bag (3) from the filter (2). The disconnected primary bag (3) is subjected to centrifugation to separate the filtered blood into blood components. The separated plasma may be transferred through a sealable, cuttable conduit (10) to a satellite bag (4). The procedure is useful for separately collecting leukocyte-removed erythrocytes, leukocyte-removed plasma, leukocyte-removed platelets, etc., from the whole blood of a healthy human.