A blood bag system comprising: a first bag configured to store whole blood or a blood component, and having an upper part and a lower part; a second bag and a third bag configured to store two blood components, respectively, among a plasma, a buffy coat, and red blood cells obtained by centrifuging a content liquid in the first bag; a first transfer tube connecting an upper part of the first bag and the second bag to each other and configured to transfer one blood component from the first bag to the second bag; and a second transfer tube connecting a lower part of the first bag and the third bag to each other and configured to transfer the other blood component from the first bag to the third bag, wherein the blood bag system further comprises: a fourth bag storing a platelet added solution; and a third transfer tube connecting an inlet of the fourth bag and a midway portion of the second transfer tube, and configured to transfer the platelet added solution from the fourth bag to the first bag storing the buffy coat, and a fifth bag configured to store a waste liquid containing the plasma remaining in the buffy coat and the platelet added solution discharged from the first bag; and the fifth bag being connected with the first bag by means of a fourth transfer tube connecting an inlet of the fifth bag and a midway portion of the first transfer tube, the fourth transfer tube configured to transfer the waste liquid discharged from the first bag to the fifth bag, the blood bag system being configured to, upon receiving of a centrifugal force acting in a transfer completed state in which the two blood components have been separately transferred to the second bag and the third bag, respectively, have the platelet added solution transferred from the fourth bag to the first bag storing the buffy coat through the third transfer tube, and to thereby discharge the plasma remaining in the buffy coat to the fifth bag, such that in the transfer completed state, the buffy coat remains in the first bag, the plasma is stored in the second bag, and the red blood cells are stored in the third bag. a first bag configured to store whole blood or a blood component, and having an upper part and a lower part; a second bag and a third bag configured to store two blood components, respectively, among a plasma, a buffy coat, and red blood cells obtained by centrifuging a content liquid in the first bag; a first transfer tube connecting an upper part of the first bag and the second bag to each other and configured to transfer one blood component from the first bag to the second bag; and a second transfer tube connecting a lower part of the first bag and the third bag to each other and configured to transfer the other blood component from the first bag to the third bag, wherein the blood bag system further comprises: a fourth bag storing a platelet added solution; and a third transfer tube connecting an inlet of the fourth bag and a midway portion of the second transfer tube, and configured to transfer the platelet added solution from the fourth bag to the first bag storing the buffy coat, and a fifth bag configured to store a waste liquid containing the plasma remaining in the buffy coat and the platelet added solution discharged from the first bag; and the fifth bag being connected with the first bag by means of a fourth transfer tube connecting an inlet of the fifth bag and a midway portion of the first transfer tube, the fourth transfer tube configured to transfer the waste liquid discharged from the first bag to the fifth bag, the blood bag system being configured to, upon receiving of a centrifugal force acting in a transfer completed state in which the two blood components have been separately transferred to the second bag and the third bag, respectively, have the platelet added solution transferred from the fourth bag to the first bag storing the buffy coat through the third transfer tube, and to thereby discharge the plasma remaining in the buffy coat to the fifth bag, such that in the transfer completed state, the buffy coat remains in the first bag, the plasma is stored in the second bag, and the red blood cells are stored in the third bag. a fourth bag storing a platelet added solution; and a third transfer tube connecting an inlet of the fourth bag and a midway portion of the second transfer tube, and configured to transfer the platelet added solution from the fourth bag to the first bag storing the buffy coat, and a fifth bag configured to store a waste liquid containing the plasma remaining in the buffy coat and the platelet added solution discharged from the first bag; and the fifth bag being connected with the first bag by means of a fourth transfer tube connecting an inlet of the fifth bag and a midway portion of the first transfer tube, the fourth transfer tube configured to transfer the waste liquid discharged from the first bag to the fifth bag, the blood bag system being configured to, upon receiving of a centrifugal force acting in a transfer completed state in which the two blood components have been separately transferred to the second bag and the third bag, respectively, have the platelet added solution transferred from the fourth bag to the first bag storing the buffy coat through the third transfer tube, and to thereby discharge the plasma remaining in the buffy coat to the fifth bag, such that in the transfer completed state, the buffy coat remains in the first bag, the plasma is stored in the second bag, and the red blood cells are stored in the third bag.