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 that can house packed red blood cells 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 porous body 63 provided on the flow passage of the transfer line 55. When the red blood cell storage solution M is transferred via the porous body 63 from the drug solution bag 54 to the master bag 50 that houses the packed red blood cells by using a difference in height between the master bag 50 and the drug solution bag 54, the porous body 63 wet with the red blood cell storage solution M prevents air from the side of the drug solution bag 54 from moving to the side of the master bag 50.