The extracorporeal blood circulating apparatus of the present invention includes: a closed-type venous reservoir having a blood storage chamber and a volume adjusting chamber that are disposed adjacently by partitioning a closed space formed by a housing; an adjusting liquid tank for storing an adjusting liquid that is connected to the volume adjusting chamber; and a blood pump that is connected to the blood storage chamber. In the housing, an inflow port for allowing blood to inflow and an outflow port for allowing blood to outflow are provided so as to communicate with the blood storage chamber, and an adjusting port for injecting and ejecting the adjusting liquid is provided so as to communicate with the volume adjusting chamber. The blood pump is connected via the outflow port, and the adjusting liquid tank is connected via the adjusting port. The closed space is partitioned by a flexible septum member so as to form the blood storage chamber and the volume adjusting chamber, and the adjusting liquid tank and the adjusting port are connected by a conduit member having a configuration that can adjust a flowing amount. Control of a blood storage amount to be most appropriate and easy adjustment are possible throughout all steps from before starting an extracorporeal blood circulation to terminating it.
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