A blood reservoir (20) for storing blood includes a first storage section (21), a second storage section (22), and a third storage section (23) that is provided between the first storage section and the second storage section, and communicates with the first storage section and the second storage section. A volume of the blood reservoir (20) can be adjusted by changing the amount of expansion or contraction of a bellows structure (28) using a bellows adjustment mechanism (83, 93).