PROBLEM TO BE SOLVED: To provide a pressure chamber which can accurately and stably detect inner pressure of a liquid circuit where fluid whose contact with the air is preferred to be avoided flows.SOLUTION: A pressure chamber 3 connected to a pressure sensor 1a includes: a body case 5 forming a blood chamber B a lid case 6 forming an air chamber A that communicates with the pressure sensor 1a a diaphragm 7 which is held between the body case 5 and the lid case 6, by which the blood chamber B and the air chamber A are partitioned, and which transmits the inner pressure of the blood chamber B to the air chamber A by elastic deformation. The body case 5 has an inner peripheral edge Ea on the blood chamber B side at a holding part 51a for holding the diaphragm 7. The diaphragm 7 is disposed while being deviated more on the air chamber A side from a virtual boundary surface F including the inner peripheral edge Ea in a membrane main part 71b surrounded by the inner peripheral edge Ea. As a result, even when the inner pressure of the blood circuit 2 becomes negative pressure, the diaphragm 7 disposed while being deviated on the air chamber A side is appropriately elastically deformed and normally functions, so that degradation of a negative pressure property can be prevented.
