It is an object of the present invention to provide a method of calibrating a pressure measurement unit for measuring the pressure inside an extracorporeal circuit while avoiding contact with the air, in which, even when the connection between an air chamber of the pressure measurement unit and a pressure measurement means is disengaged accidentally or intentionally after the operation of changing the pressure inside the extracorporeal circuit from the initial pressure, the pressure measurement unit can be reconnected and the pressure inside the extracorporeal circuit can be measured stably. After closing the extracorporeal circuit in such a way that the pressure measurement unit and a pressure adjusting means are contained in the closed section, the pressure inside the liquid chamber is adjusted to a predetermined pressure using a pressure adjusting pump, and then the pressure measurement means and the air chamber of the pressure measurement unit are connected to each other.
