PROBLEM TO BE SOLVED: To provide a pressure chamber which can prevent a seal member from projecting and stably maintain airtightness.SOLUTION: A pressure chamber 3 includes: a container body 4 connected to a tube 2a diaphragm 7 which divides a blood chamber B where blood flows and an air chamber A inside the container body 4 and transmits inner pressure in the blood chamber B to the air chamber A by elastic deformation a connection tube 62 which protrudes from the container body 4, is inserted in a pressure receiving port 16, and communicates with the air chamber A an O ring 63 which is placed on an outer periphery of the connection tube 62 and is pressed against an end part 16a of the pressure receiving port 16 and a circular rib 64 which encircles the O ring 63 by protruding from the container body 4 in a circular shape and covers the pressure receiving port 16 beyond the end part 16a of the pressure receiving port 16 when the pressure chamber 3 is mounted on the pressure receiving port 16.
