An apparatus for extracorpo real blood treatment, comprising a treatment unit (2) having a first chamber (3) and a sec ond chamber (4) separated from one another by a semipermeable membrane (5), a blood removal line (6) connected in inlet with the first chamber (3) and a blood return line (7) connected in outlet with the first chamber an infusion line (99a, 9b) of a replacement fluid and a fluid evacuation line (10) connected in outlet from the second chamber. A regulating device (20) of a transmembrane pressure is active on at least one of the lines and a con trol unit (15) is configured to: command the regulating device (20) by setting a first in crease (STMPi), determine a value of a con trol parameter (q>i) corresponding to the first increase, compare the value of the control p a rameter (q>i) with a reference value ( ) and, if the value of the control parameter is greater than the reference value, commandthe regu lating device (20) by setting a second in crease (STMP 2) which is greater than the first o FIG.3 increase (STMPi).
