PROBLEM TO BE SOLVED: To provide a blood purifying apparatus which can prevent an excessive condensation to the blood which extracorporeally circulates from occurring when the on-line HDF or the on-line HF of a replacement fluid is performed.SOLUTION: The blood purifying apparatus includes: a dialyzer 1 an artery-side blood circuit 2 a vein-side blood circuit 3 a dialysate introducing line L1 a dialysate discharging line L2 which discharges a dialysate from the dialyzer 1 a replacement fluid line L3 and a replacement fluid pump 9 which can feed the dialysate having flown in the replacement fluid line L3 to the artery-side blood circuit 2. The blood purifying apparatus is equipped with a control means 10 which can perform the control in such a manner that the flow rate of the dialysate which is introduced to the dialyzer 1 may be made approximately the same as the flow rate of the dialysate which is discharged from the blood purifying implement until a specified period of time has passed from the starting of the feeding of the dialysate to the artery-side blood circuit 2 by the replacement fluid pump 9 on condition that the blood flowing in a dilution flow channel section A is not diluted with the dialysate.
