PROBLEM TO BE SOLVED: To provide a blood component concentration measuring device capable of correctly and stably measuring the concentration of blood components in a blood purification circuit.SOLUTION: The blood component concentration measuring device 1 has: a sensor 60 having a light-emitting unit for making a light incident on blood in the blood purification circuit 20 and a light-receiving unit for detecting a light having transmitted through the blood in the blood purification circuit 20； and a calculator 72 for extracting a frequency component of the frequency F equal to the pulse of the blood generated in the blood purification circuit 20 by driving a blood pump 30 on the basis of variation in time of optical strength detected by the light-receiving unit and calculating oxygen concentration on the basis of the frequency component. The sensor 60 is provided on the blood purification circuit 20 so as to be positioned downstream of the blood pump 30.
