A third cassette (28) of a blood component collection kit (12) is equipped with a third cassette main body (29) in which a flow path (F) is formed together with having a filter accommodating unit (86), and a filter member (88) disposed in the filter accommodating unit (86). The flow path (F) includes a red blood cell line (f1) in which red blood cells flow, a preservation solution line (f2) in which a preservation solution flows, a confluence section (f3) in which the red blood cells and a preservation solution are made to merge together, a mixed liquid line (f4) through which a mixed liquid of the red blood cells and the preservation solution flows, and a filter chamber (f5) formed inside the filter accommodating unit (86).
