A blood component sampling cassette which can be more efficiently manufactured at lower cost as compared to a typical cassette, a blood sampling circuit set, and a blood component sampling system. A blood component sampling cassette (22) includes a cassette main body (23) made of a soft material to which heat sterilization is applicable. The cassette main body (23) is provided with a retransfusion line (44). The retransfusion line (44) is provided with a reservoir (47) configured to temporarily store a blood component to be returned to a blood donor. The reservoir (47) is pressed by a retransfusion pump (49) to discharge the blood component from the reservoir (47).