[Problem] To provide a blood purification device which can ensure the replacement of blood when the blood is returned, and which enables the reduction in the amount of a replacement fluid to be supplied.[Solution] A blood purification device provided with a blood circuit which comprises an artery-side blood circuit (1) and a vein-side blood circuit (2) and can circulate blood of a patient extracorporeally from the tip end of the artery-side blood circuit (1) to the tip end of the vein-side blood circuit (2) and a dialyzer (3) which is arranged between the artery-side blood circuit (1) and the vein-side blood circuit (2) and can purify the blood, in which the blood in the blood circuit can be replaced when the blood is returned after the completion of a therapy. The blood purification device is also provided with an air flow line which is connected to a predetermined part in the blood circuit and through which air can flow, an air pump (18) which can supply air to the blood circuit through the air flow line, and a control means (19) which can control the air pump (18) so as to supply air to the blood circuit when the blood is returned.Linvention fournit un dispositif de purification de sang qui tout en permettant une permutation sûre de sang lors dune retransfusion, permet de modérer la quantité de sang de permutation acheminée. Plus précisément, linvention concerne un dispositif de purification de sang qui permet de permuter le sang à lintérieur dun circuit de sang lors de la retransfusion après un traitement, et qui est muni : du circuit de sang qui est constitué dun circuit de sang côté artère (1) et dun circuit de sang côté veine (2), et qui permet une circulation extracorporelle du sang du patient depuis lextrémité avant du circuit de sang côté artère (1) jusquà lextrémité avant du circuit de sang côté artère (1) et dun dialyseur (3) qui est placé entre le circuit de sang côté artère (1) et le circuit de sang côté veine (2), et qui purifie le sang. Le dispositif
