A biological component treatment system (22) includes a biological component kit (12) having tubes (16) that allow a liquid to flow therethrough and a biological component cassette (10), and a biological component treatment device (14). The biological component cassette (10) includes flow paths (44) in the interior thereof through which the liquid is allowed to flow, and is equipped with a cassette main body (40) formed in a sheet shape that possesses flexibility. The cassette main body (40) includes a liquid level target detection unit (80) that causes a liquid level of the liquid to be detected. A cross-sectional area of a storage space (80a) of the liquid level target detection unit (80) is greater than a cross-sectional area of the flow paths (44).
