A closed-type blood reservoir comprising:an outer shell in which a blood storage chamber shell (11a) and a volume adjustment chamber shell (11b) each including a curved, outwardly convex portion are joined together and form a space therewithina flexible septum (3) that is interposed between the blood storage chamber shell (11a) and the volume adjustment chamber shell (11b), and that divides the space into a blood storage chamber (1) for storing blood and a volume adjustment chamber (2) for storing volume adjustment liquida blood inlet port (4) , a blood outlet port (5), and a blood storage chamber air discharge port (9), that are provided in the blood storage chamber shell (11a) anda volume adjustment liquid port (8) for injecting and ejecting the volume adjustment liquid (2) and a volume adjustment chamber air discharge port (10) that are provided in the volume adjustment chamber shell (11b)wherein part of the inner circumferential portion region of the septum (3) along the outer circumferential edge of the blood storage chamber shell (11a) forms a flat portion that is substantially flat, and the inner region of the septum (3) that is located inside the flat portion is molded such that it can project as a curved surface toward the blood storage chamber shell (11a) or the volume adjustment chamber shell (11b).
