A blood purification apparatus to which a blood circuit that allows a patient's blood to extracorporeally circulate and a blood purifier connected to the blood circuit and being configured to purify the blood in extracorporeal circulation are attachable, the blood purification apparatus including a dialysate introduction line through which dialysate can be introduced into the blood purifier ; a dialysate drain line through which waste dialysate resulting from blood purification performed by the blood purifier can be drained from the blood purifier ; and a concentration-detecting unit that is configured to detect a concentration of a predetermined substance in the waste dialysate resulting from the blood purification by the blood purifier and flowing through the dialysate drain line , characterized in that the blood purification apparatus comprises : a control unit that is configured to establish a state of equilibrium where the concentration of the predetermined substance in the waste dialysate flowing through the dialysate drain line and a concentration of the predetermined substance in the blood flowing through the blood circuit are equal or approximate to each other; a storage unit configured to storing a value detected by the concentration-detecting unit in the state of equilibrium as an equilibrium value; and a clearance-calculating unit that is configured to calculate clearance in accordance with the value detected by the concentration-detecting unit and the equilibrium value stored in the storage unit , the clearance being a figure of merit representing a degree of solute removal by the blood purifier . a dialysate introduction line through which dialysate can be introduced into the blood purifier ; a dialysate drain line through which waste dialysate resulting from blood purification performed by the blood purifier can be drained from the blood purifier ; and a concentration-detecting unit that is configured to detect a concentration of a predetermined substance in the waste dialysate resulting from the blood purification by the blood purifier and flowing through the dialysate drain line , characterized in that the blood purification apparatus comprises : a control unit that is configured to establish a state of equilibrium where the concentration of the predetermined substance in the waste dialysate flowing through the dialysate drain line and a concentration of the predetermined substance in the blood flowing through the blood circuit are equal or approximate to each other; a storage unit configured to storing a value detected by the concentration-detecting unit in the state of equilibrium as an equilibrium value; and a clearance-calculating unit that is configured to calculate clearance in accordance with the value detected by the concentration-detecting unit and the equilibrium value stored in the storage unit , the clearance being a figure of merit representing a degree of solute removal by the blood purifier . a control unit that is configured to establish a state of equilibrium where the concentration of the predetermined substance in the waste dialysate flowing through the dialysate drain line and a concentration of the predetermined substance in the blood flowing through the blood circuit are equal or approximate to each other; a storage unit configured to storing a value detected by the concentration-detecting unit in the state of equilibrium as an equilibrium value; and a clearance-calculating unit that is configured to calculate clearance in accordance with the value detected by the concentration-detecting unit and the equilibrium value stored in the storage unit , the clearance being a figure of merit representing a degree of solute removal by the blood purifier . <EOS>