Provided are a blood bag system and a blood treatment method capable of securely and efficiently obtaining a medium specific gravity blood component having a sufficiently low content rate of a light specific gravity blood component.In a blood bag system (10) used for a blood treatment method, in a transfer completed state in which whole blood in a first bag (12) is centrifuged to transfer blood plasma to a second bag (14) through a first transfer tube (24) and transfer red cell concentrates to a third bag (16) through a second transfer tube (26), a buffy coat is left in the first bag (12). In the transfer completed state, when a platelet added solution is transferred from a fourth bag (18) to the first bag (12) on which a centrifugal force acts through a third transfer tube (28), blood plasma in the buffy coat is discharged from the first bag (12).
