A blood purification apparatus includes a blood purification instrument for extracorporeally circulating blood of a patient and a concentration detector detecting a concentration of liquid flowing during blood purification. The concentration detector has a light emitter irradiating light onto said liquid, a light receiver receiving light from the light emitter transmitted through said liquid, and a detector detecting the received light intensity received by the light receiver. The concentration detector detects the concentration of the liquid based on the received light intensity and can be calibrated by adjusting an amount of irradiation by the light emitter so that the received light intensity has a predetermined value.
