An apparatus (1) for extracorporeal blood treatment comprising a blood treatment device (2) having a semipermeable membrane (5) which separates a blood chamber (3) from a fluid chamber (4). A used fluid line (7) connects the fluid chamber to a used fluid discharge. A first pump (8) controls the ultrafiltration through the semipermeable membrane. A second pump (9) arranged in the used fluid line downstream of the first pump keeps pressure between the first and the second pump at a constant predetermined value. The invention provides a protection system against any possible pressure change in the used fluid discharge in a hemodialysis or hemo(dia)filtration apparatus.