[object] Disclosed is a blood purification apparatus capable of preventing excess concentration in blood circulating extracorporeally during online HDF or online HF performing a pre-substitution. [Solution] The blood purification apparatus is equipped with a dialyzer 1, an arterial blood circuit 2 in which a blood pump 4 has been disposed, a venous blood circuit 3, a dialysate introduction line L1 that introduces a dialysate to the dialyzer 1, a dialysate discharge line L2 that discharges the dialysate from the dialyzer 1, a substitution line L3, and a substitution pump 9 that supplies the dialysate flowing in the substitution line L3 to the arterial blood circuit 2, wherein a control device 11 is provided capable of estimating or measuring the concentration of blood in a dilution channel section A； on the arterial blood circuit 2 that includes the fluid channel spanning from the connection site of the substitution line L3 to the dialyzer 1, and that controls the volume of dialysate supplied by the substitution pump 9 on the basis of the estimated or measured blood concentration.
