The blood bag system (100) includes a first bag (101) for storing blood sampled from a donor, a first tube (106a) for transferring blood from the first bag (101), a filter (21) for reduced blood transferred from the first tube (106a), a second tube (106b) for transferring filtered blood, and a second bag (102) for storing filtered blood transferred from the second tube (106b). The first bag (101) contains a predetermined amount of air satisfying an equation: (V1+V2+V3)<(amount of air)<(V1+V2+V3+V4+V5), where V1 is a volume in the first tube, V2 is a volume of the blood inflow chamber, V3 is a void volume of the filter, V4 is a volume of the blood outflow chamber, and V5 is a volume in the second tube.
