PROBLEM TO BE SOLVED: To provide a blood purifier capable of safely returning blood without the need of reversing a blood pump, and automating the whole step of returning the blood without other additional pumps.SOLUTION: A blood purifier includes: a blood purification element 7 a blood pump 8 an infusion line 11 connected to an artery-side blood circuit 3 a first choking means 13 a second choking means 14 and third choking means 15. The blood pump has a spacing control means capable of controlling an interval between a tube receiver and a roller, and controlling the speed of changing the interval therebetween. A blood returning step control means is provided which executes: a vein-side blood returning step in which blood in the blood circuit is pushed out with the an infusion and returned a pressure accumulation step in which infusion sending operation is executed so as to introduce the infusion into the blood circuit by sending operation of the blood pump with the first and second choking means closed, and the pressure of the infusion is raised between the second choking means and the blood pump in the blood circuit and an artery-side blood returning step in which the blood is returned upstream by closing the third choking means, opening the first choking means, stopping the sending operation of the blood pump, releasing a tube from its stoppage, and relieving the pressure of the infusion.
