PROBLEM TO BE SOLVED: To prevent a great deformation or damage of a filter and to appropriately purify blood components.SOLUTION: A blood separation system 1 has: a blood bag circuit 10 in which a filter 25 is arranged in a flow path 23 connecting the blood bag 20 and a blood component bag 21 and a centrifugal separator 11 on which the blood bag circuit 10 can be mounted, and which applies a centrifugal force to the blood bag circuit 10, separates blood in the blood bag 20 by centrifugation, and sends the centrifuged blood components to the blood component bag 21 through the filter 25. A mounting part 54 of the filter 25 is arranged on the axis C of the centrifugal force.