A blood bag system includes: a first bag in which blood containing multiple components is stored; a second bag in which a relative medium-density component is stored; a third bag in which a relative low-density component is stored; a first tube through which the relative low-density component and the relative medium-density component are transported from the first bag to the second bag; and a second tube through which the relative low-density component is transported from the first bag to the third bag. A blood treatment method involves: centrifuging blood containing multiple components in a first bag, transporting a low-density component from the first bag to a third bag through a first tube and transporting a relative medium-density component from the first bag to the second bag through the first tube.