A blood purification apparatus comprising a blood purification instrument for extracorporeally circulating blood of a patient and a concentration detecting means for detecting a concentration of liquid flowing during blood purification, wherein the concentration detecting means comprises: a light emitting means for irradiating light onto said liquid, a light receiving means for receiving light from the light emitting means transmitted through said liquid, a detecting means for detecting the received light intensity received by the light receiving means, a light emission control means for controlling an amount of light emitted by the emitting means, a deciding means for deciding whether the received light intensity detected by the detecting means is substantially same as a predetermined value, and an adjusting means for making the light emission control means control the amount of light so that the received light intensity has the predetermined value based on the decision of the deciding means wherein the concentration detecting means is adapted to detect the concentration of the liquid based on the received light intensity detected by the detecting means, and the concentration detecting means is adapted to be calibrated by adjusting an amount of irradiation by the light emitting means so that the received light intensity received by the light receiving means has a predetermined value, characterized in that, the concentration detecting means is a discharged liquid concentration sensor that can monitor the blood purification efficiency by detecting the concentration of the liquid discharged from the blood purification instrument, and the blood purification apparatus is configured to perform the calibration of the concentration detecting means prior to the blood purification treatment and prior to extracorporeally circulating the blood of a patient by supplying dialysate to the blood purification instrument and discharging the dialysate from the blood purification instrument during the calibration. a light emitting means for irradiating light onto said liquid, a light receiving means for receiving light from the light emitting means transmitted through said liquid, a detecting means for detecting the received light intensity received by the light receiving means, a light emission control means for controlling an amount of light emitted by the emitting means, a deciding means for deciding whether the received light intensity detected by the detecting means is substantially same as a predetermined value, and an adjusting means for making the light emission control means control the amount of light so that the received light intensity has the predetermined value based on the decision of the deciding means wherein the concentration detecting means is adapted to detect the concentration of the liquid based on the received light intensity detected by the detecting means, and the concentration detecting means is adapted to be calibrated by adjusting an amount of irradiation by the light emitting means so that the received light intensity received by the light receiving means has a predetermined value, characterized in that, the concentration detecting means is a discharged liquid concentration sensor that can monitor the blood purification efficiency by detecting the concentration of the liquid discharged from the blood purification instrument, and the blood purification apparatus is configured to perform the calibration of the concentration detecting means prior to the blood purification treatment and prior to extracorporeally circulating the blood of a patient by supplying dialysate to the blood purification instrument and discharging the dialysate from the blood purification instrument during the calibration.