Disclosed is a method for separating a blood material into leukocyte-removed blood component products, which comprises providing a multiple blood bag system comprising a primary bag containing a blood material, a leukocyte-removing filter device comprising a flat casing, and at least one satellite bag, and centrifuging the blood material using a centrifuge cup having the multiple blood bag system accommodated therein, to thereby separate the blood material into blood components, wherein the centrifugation is performed with the filter device being held by a filter device holder having a substantially flat dimension which is disposed in a centrifuge cup so as to extend in a direction perpendicular to an inner bottom floor of the centrifuge cup, and wherein the filter device holder has a recess and the filter device is received by the recess in a manner such that a flat surface of the flat casing of the filter device is perpendicular to the inner bottom floor of the centrifuge cup. By the method of the present invention, not only can the accommodation of a multiple blood bag system into a centrifuge cup be readily performed without any additional cumbersome operations, but also the danger of damaging the filter device and the blood bags is extremely reduced or eliminated.