An insert unit to be inserted into an unit insertion hole of a centrifuging means, and a blood bag system, the blood bag system mounted in the insert unit and comprising: a first bag configured to store blood containing a plurality of components; a second bag configured to store a relative medium-density component obtained by centrifugation of the blood in the first bag; a third bag configured to store a relative low-density component obtained by centrifugation of the blood in the first bag; a first tube through which an upper portion of the first bag and an inlet of the second bag are connected and configured to transport there through the relative low-density component and the relative medium-density component from the first bag to the second bag, the inlet of the second bag being provided on one end of the second bag; and a second tube through which an outlet of the second bag, which is provided on the other end of the second bag, and the third bag are connected and configured to transport there through the relative low-density component from the first bag to the third bag by way of the first tube and the second bag, the insert unit comprising an unit main body being a bottomed tube which has a wide arcuate shape in top plan view and is opening at the top, wherein a first chamber on the inside diameter side and a second chamber on the outside diameter side are partitioned from each other by an arcuate wall, and a cover body, wherein the blood bag system is configured to be mounted into the insert unit by - holding the first and second tubes (18, 20) on a tube holder; - containing the first bag into the first chamber of the unit main body; - containing the second bag and the third bag into the second chamber; wherein the second bag is inserted such that the inlet is provided at an upper portion thereof, the first tube being connected to an upper portion of the second bag, and the second tube being connected to a lower portion of the second bag. a first bag configured to store blood containing a plurality of components; a second bag configured to store a relative medium-density component obtained by centrifugation of the blood in the first bag; a third bag configured to store a relative low-density component obtained by centrifugation of the blood in the first bag; a first tube through which an upper portion of the first bag and an inlet of the second bag are connected and configured to transport there through the relative low-density component and the relative medium-density component from the first bag to the second bag, the inlet of the second bag being provided on one end of the second bag; and a second tube through which an outlet of the second bag, which is provided on the other end of the second bag, and the third bag are connected and configured to transport there through the relative low-density component from the first bag to the third bag by way of the first tube and the second bag, the insert unit comprising an unit main body being a bottomed tube which has a wide arcuate shape in top plan view and is opening at the top, wherein a first chamber on the inside diameter side and a second chamber on the outside diameter side are partitioned from each other by an arcuate wall, and a cover body, wherein an unit main body being a bottomed tube which has a wide arcuate shape in top plan view and is opening at the top, wherein a first chamber on the inside diameter side and a second chamber on the outside diameter side are partitioned from each other by an arcuate wall, and a cover body, wherein the blood bag system is configured to be mounted into the insert unit by - holding the first and second tubes (18, 20) on a tube holder; - containing the first bag into the first chamber of the unit main body; - containing the second bag and the third bag into the second chamber; wherein the second bag is inserted such that the inlet is provided at an upper portion thereof, the first tube being connected to an upper portion of the second bag, and the second tube being connected to a lower portion of the second bag. - holding the first and second tubes (18, 20) on a tube holder; - containing the first bag into the first chamber of the unit main body; - containing the second bag and the third bag into the second chamber; wherein the second bag is inserted such that the inlet is provided at an upper portion thereof, the first tube being connected to an upper portion of the second bag, and the second tube being connected to a lower portion of the second bag.