PROBLEM TO BE SOLVED: To provide a blood purifier which can perform actions and operations according to the final stage of reinfusion.SOLUTION: A blood purifier comprises: a blood circuit that may extracorporeally circulates blood of a patient from a tip end of an arterial blood circuit 1 to a tip end of a venous blood circuit 2 a dialyzer 3 to purify blood flowing through the blood circuit and substitution solution supply means capable of supplying a substitution solution into the blood circuit. The blood purifier performs reinfusion by substituting the blood in the blood circuit with the substitution solution supplied by the substitution solution supply means after treatment. The blood purifier further comprises: blood determination means 13, 14 arranged at a predetermined position in the arterial blood circuit 1 or venous blood circuit 2 and may detect presence or absence of the blood flowing in the arterial blood circuit or venous blood circuit at the predetermined position and recognition means 17 that may recognize a final stage of the reinfusion which is in a condition near the end of the substitution with the substitution solution on the basis of the presence or absence of the blood detected by the blood detection means 13, 14 during the reinfusion.SELECTED DRAWING: Figure 1
