PROBLEM TO BE SOLVED: To provide a blood purifier capable of reliably preventing bubbles from entering blood purifier in a liquid replenishing line priming step.SOLUTION: The blood purifier has a control means 16 capable of discharging bubbles from an overflow line L10 by simultaneously driving a blood pump 4 and a liquid replenishing pump 13 while setting a driving speed of the blood pump 4 greater than that of the liquid replenishing pump 13, in the process of the liquid replenishing line priming step for filling a replenishing line L8a with dialyzing liquid with the distal end of an artery-side blood circuit 2 and the distal end of a vein-side blood circuit 3 coupled to each other.
