Provided is a blood purification device whereby an error in detecting concentration caused by a temperature change can be prevented and thus the accuracy in detecting the concentration of a liquid using a concentration-detecting means can be increased. The blood purification device is provided with a dialyzer (2) for purifying the blood of a patient through extracorporeal circulation and a drainage concentration sensor (5) for detecting the concentration of a liquid flowing in association with the blood purification, wherein said drainage concentration sensor (5) is provided with a light emitting means (11) which can irradiate the drainage with light, a light receiving means (12) which can receive transmitted light from the light emitting means (11), said transmitted light having passed the drainage, a referential light-receiving means (15); which can receive referential light from the light emitting means (11), said referential light having diverged without passing through the drainage, a detecting means (13) which can detect the received light intensities of the transmitted light and referential light respectively, and a correcting means (16) which can absorb a detection error made by the aforesaid drainage concentration sensor (5) based on the light intensity of the referential light received by the referential light-receiving means (15).