A method characterized in that, when calibrating a measured value of a level in blood flowing through a pipe having light-permeable walls on the basis of Lambert-Beer's law, the blood flowing through the pipe passes through a water removal device, and the measured value is calibrated on the basis of the difference in the measured values including the offset level of the blood levels before and after water removal by the water removal device, and the amount of blood flowing through the pipe before and after water removal by the water removal device.
