An apparatus for extracorporeal blood treatment, comprising a treatment unit (2) having a first chamber (3) and a second 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 (9; 9a, 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 control unit (15) is configured to: command the regulating device (20) by setting a first increase (δTMP1), determine a value of a control parameter (φ1) corresponding to the first increase, compare the value of the control parameter (φ1) with a reference value (φref) and, if the value of the control parameter is greater than the reference value, command the regulating device (20) by setting a second increase (δTMP2) which is greater than the first increase (δTMP1).