A blood purification apparatus comprising a blood purification instrument for extracorporeally circulating blood of a patient and a concentration detecting means adapted to detect 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, characterized in that the concentration detecting means further comprises: 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 of the reference light received by the reference light receiving means, and in 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, wherein the error absorbing means comprises a correcting means for correcting a current supplied to the light emitting means so that a light amount irradiated by the light emitting means is constant with the passage of time based on the received light intensity of the reference light received by the reference light receiving means, wherein 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. 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, characterized in that the concentration detecting means further comprises: 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 of the reference light received by the reference light receiving means, and in that 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 of the reference light received by the reference light receiving means, and in 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, wherein the error absorbing means comprises a correcting means for correcting a current supplied to the light emitting means so that a light amount irradiated by the light emitting means is constant with the passage of time based on the received light intensity of the reference light received by the reference light receiving means, wherein 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.