An antithrombotic blood treating system comprising a combination of a permselective hollow fiber membrane containing a copolymer having a polyethylene oxide unit; a header having a blood-contacting surface coated with either a heparinized hydrophilic copolymer containing vinyl chloride as the constituent or a hydrophilic copolymer having a polyethylene oxide unit; a catheter; and a circuit. It becomes possible for the first time to conduct extracorporeal circulation for a long period of time without the necessity for internally administering heparin by imparting an antithrombotic property to the entire blood-contacting surface.
