Dialysis apparatus comprising a blood treatment unit (5) having a dialysate chamber (13) and a blood chamber (12) separated by a semipermeable membrane (14), a dialysate circuit (32) presenting a supply line (2) and a discharge line (6) connected to said blood treatment unit (5), a preparation device (33) for preparing a dialysate containing a substance present in the blood too and comprising regulating means (38) for regulating the concentration of said substance in the dialysate. A blood circuit (8) is configured to circulate extracorporeal blood through said blood chamber (12)； control means (19) determines a value representative of the concentration of said substance in the blood and are programmed for driving said regulating means (38) as a function of the determined value representative of the substance concentration in the blood such that the substance concentration in the dialysate tends towards the substance concentration in the blood.
