PROBLEM TO BE SOLVED: To provide a blood bag system that can efficiently produce a blood product.SOLUTION: A blood bag system 10 comprises: a master bag 50 a drug solution bag 54 that houses a red blood cell storage solution M a transfer line 55 that forms a flow passage between the master bag 50 and the drug solution bag 54 and a plug mechanism 63 provided on the flow passage of the transfer line 55 and including a plug body 64 having buoyancy with respect to the red blood cell storage solution M. When the red blood cell storage solution M is transferred from the drug solution bag 54 to the master bag 50 by using a difference in height between the master bag 50 and the drug solution bag 54, the plug body 64 moves in the flow passage and blocks the flow passage.
