A first ray of light having a wavelength absorbed by a specific pigment which is put into the blood of a living body tissue and ingested and excreted by the liver, and a second ray of light having a wavelength not absorbed by the pigment are radiated from light sources (11,12) to the living body tissue (15). The optical pulses obtained from the living body tissue are received (13) and its light reception output is sampled and converted to a digital signal by an A/D converter (30). The variable component in the blood contained in the sampling output is determined as a linear regression coefficient to make living body calibration. After the specific pigment is injected, a value associated with a specific pigment concentration in the blood is calculated on the basis of the output within a predetermined period of time and coefficient thus determined. The coefficient of a simulation function in terms of the coefficient of time is determined on the basis of the value described above and by the method of least squares. The disappearance ratio and retention ratio of plasma are then determined on the basis of this coefficient.
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