A blood purification apparatus has a dialyzer (1), an arterial blood circuit (2) with a blood pump 4, a venous blood circuit 3, a dialysate introduction line L1, a dialysate discharge line L2, 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. A control device (11) estimates or measures the concentration of blood in a dilution channel section A. The control device (11) controls the volume of dialysate supplied by the substitution pump 9 on the basis of the estimated or measured blood concentration.