An object of the present invention is to provide a blood purification apparatus that can improve the detecting accuracy of liquid concentration and, thus, the reliability of the concentration detecting means. According to the present invention, there is provided 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 characterized in that 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, and that the concentration detecting means can detect the concentration of the liquid based on the received light intensity detected by the detecting means and can 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.
