Disclosed is a blood bag system (10) including: a first bag (12) in which blood containing multiple components is stored a second bag (14) in which a relative medium-density component is stored a third bag (16) in which a relative low-density component is stored a first tube (18) through which the relative low-density component and the relative medium-density component are transported from the first bag (12) to the second bag (14) and a second tube (20) through which the relative low-density component is transported from the first bag (12) to the third bag (16). Also disclosed is a blood treatment method including: a centrifugation step of centrifuging blood containing multiple components in a first bag (12) and a separation step of transporting a relative low-density component from the first bag (12) to a third bag (16) through a first tube (18) and transporting a relative medium-density component from the first bag (12) to the second bag (14) through the first tube (18).
