Abstract: A blood reservoir serves to remove air bubbles from blood and maintain a desired amount of blood in an extracorporeal blood circulation circuit. The blood reservoir has a blood inlet chamber having blood inlet ports, a blood storage chamber having a blood outlet port, and a blood debubblizer disposed between the blood inlet chamber and the blood storage chamber and extending the full width across the blood passage defined by the blood inlet chamber. A rib is held against the side of the debubblizer closer to the blood storage chamber and extends the full width across the blood passage, the rib having a certain height. Blood introduced into the blood reservoir is forced by the rib to stay in the debubblizer, during which time air bubbles are reliably removed from the blood by the debubblizer.