A device for protecting medical apparatus from contamination by infectious agents comprises a containment body having an inlet destined to be set in fluid communication with an extracorporeal circuit of blood, and an outlet destined to be connected to a fluid line operatively connected to a pressure gauge of a medical apparatus. The inlet is in gas communication with the outlet across an internal cavity of the containment body. Two hydrophobic membranes are predisposed in the containment body between the inlet and the outlet. The membranes each define an anticontamination barrier which is gas-permeable. The device transmits the pressure of the extracorporeal circuit to the pressure gauge, with no relevant loss of head, while at the same time protecting, with a high degree of security, the medical apparatus from risks of contamination by pathogens originating in the extracorporeal circuit.