The present invention provides an apparatus for evaluating biological function that, in differentiating as far as possible information from the capillaries, which reflects tissue metabolism, from information from outside the tissue (for example, the arteries and veins), has high speed and accuracy enabling it to compensate for the low spatial resolution of conventional near infrared spectroscopy methods, and that furthermore does not merely monitor changes in oxygen concentration and the like, but makes it possible to easily and conveniently distinguish between capillary reactions, metabolic reactions and the like; its configuration is characterized in that it provides a light irradiation means for irradiating light to a specified site of a living body, a light detection means for detecting light exiting from the living body, a calculation means for determining the respective changes in concentration of oxygenated hemoglobin and deoxygenated hemoglobin by performing calculations in near infrared spectroscopy with the intensity of the detected light as a parameter, and a display means for displaying information concerning the relative ratio "k" between both these data over time.