An object of the present invention is to provide a blood purification apparatus, which can minimize the detecting error of liquid concentration caused by temperature variation and thus improve the accuracy of detection of liquid with a 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 reference light receiving means for receiving reference light branched from the light emitting means without being transmitted through said liquid, and a detecting means for detecting the light intensity respectively of the transmitted light received by the light receiving means and the reference light received by the reference light receiving means, and that the blood purification apparatus further comprises an error absorbing means for detecting the concentration of the liquid based on the received light intensity of the transmitted light received by the light receiving means and detected by the detecting means and for absorbing a detected error of the concentration detecting means based on the received light intensity of the reference light received by the reference light receiving means.
