A liver function testing apparatus, provides light signals (11, 12) exposing vital tissue (15) to a first light signal that is substantially absorbed by a specific dye dosed into the blood of the vital tissue. The dye is to be taken in and removed by the liver. A second light signal is substantially not absorbed by the specific dye. Optical pulses obtained from the vital tissue are received by a light receiving element (13), the output of which is sampled by an A-D converter (30) to provide digital output signals. On the basis of variable components in the blood, a CPU (34) determines a coefficient of a regression line expression between first and second photoelectric conversion signals, to perform a biocalibration. A value correlated with a specific dye concentration in the blood, is processed on the basis of sampling signals during a prescribed period after an injection of the specific dye and the determined coefficient of the regression line expression, so that a coefficient of a simulation function is obtained as a function of time using the method of least squares. Additionally, an index RMAX expressing the total amount of hepatic cell function is obtained on the basis of the coefficients.
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