A biological component treatment system (22) includes a biological component kit (12), and a biological component treatment device (14) in which the biological component kit (12) is set. The biological component kit (12) includes tubes (16) and a biological component cassette (10). The biological component cassette (10) includes a cassette main body (40) having flow paths (44) in the interior of the cassette main body (40), and the cassette main body (40) includes flow path opening/closing units (100) which are capable of switching between opening and closing of the flow paths (44). The flow path opening/closing units (100) include cutouts (102) provided at positions adjacent to the flow paths (44).
