A collecting device (1) for fluids for medical use, comprising: a rigid container (2) internally defining a first chamber (2a) having a fixed volume, an auxiliary container (3) having a deformable portion (4) defining internally thereof a second chamber (3a) having a variable volume as a function of a deformable portion thereof, a passage (5) able to set the first and the second chamber (2a, 3a) in fluid communication, an inlet port (6) connected to at least one of the containers and an outlet port (7) connected to at least one of the containers, as well as a bypass line (24) for bypassing the device (1).
