A removable pressure sensor assembly senses a pressure in an elastically deformable tube conveying blood in an extracorporeal circulator. The assembly includes a main body portion 31 and a pressure measurement element 40 disposed in main body portion 31. Main body portion 31 has a base portion 32with a tube mounting recessed portion 34 such that an intermediate part of a tube 11 is removably fitted such that tube 11 is elastically deformed. Pressure measurement element 40 is exposed to tube 11 so that it measures a pressure of blood inside tube 11. A lid portion 33 holds tube 11 inside tube mounting recessed portion 34 by selectably closing recessed portion 34 of the base portion. Tube mounting recessed portion 34 has a rectangular cross section, and a width L of the rectangular cross section is set to be smaller than an external dimension D of tube 11.
