A blood circuit having a pressure measurement portion connected to a pressure measurement device (3,6, 12) includes: a first measurement portion (5, 11) provided downstream of a pumping segment (4)； and a second measurement portion (2) provided upstream of the pumping segment. The first measurement portion (5, 11) includes: a first housing having a tubular shape； and a first flexible membrane having a tubular shape and provided in the first housing. The second measurement portion (2) includes: a second housing having a tubular shape； and a second flexible membrane having a tubular shape and provided in the second housing. A space between the first flexible membrane and the first housing is larger than a space between the second flexible membrane and the second housing in an initial state before permitting the blood to flow.
