An initial flow blood bag (10) that configures a blood bag system (12) includes a bag body (80) that is formed of a soft material in a bag-shaped manner, and can house initial flow blood. The bag body (80) includes a first housing portion (84) provided with a blood inflow portion (82), a second housing portion (92) provided with a blood outflow portion (94), and a communication path (90) configured from partitions (86, 88) that partition an inside of the bag body (80) into the first housing portion (84) and the second housing portion (92), and allowing the first housing portion (84) and the second housing portion (92) to communicate with each other.
