A blood circuit having a pressure measurement portion connected to a pressure measurement device includes: a first measurement portion provided downstream of a pumping segment; and a second measurement portion provided upstream of the pumping segment. The first measurement portion 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 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.