An apparatus for evaluating biological function of a living body comprising a plurality of living body probes provided with light-emitting elements for irradiating light to specified sites of a living body and light-receiving elements for receiving and detecting light exiting the living body, a behavioral information measuring part for measuring behavioral information of said living body, and an apparatus body for receiving light information detected by said living body probe and behavioral information measured by said behavioral information measuring part and adapted to perform calculation, control and memory operations, and to use near-infrared spectroscopy to evaluate biological function, said apparatus body comprising: a controller configured to obtain, based on light information from said living body probe, by means of time series measurements, a first two-dimensional diagram plotting on polar coordinates loci showing amounts of change in oxyhemoglobin versus amounts of change in deoxyhemoglobin, and a second two-dimensional diagram plotting on polar coordinates loci showing absolute amounts of oxyhemoglobin versus absolute amounts of deoxyhemoglobin, wherein the axes of said first and second two-dimensional diagrams are inclined by 45 degrees with regards to one another; said controller being further configured to derive from said first and second two-dimensional diagrams, a variety of parameters, based on a theory assuming the oxygen exchange between oxyhemoglobin and deoxyhemoglobin in hemodynamics to be rotational motion and establishing within the equation of rotational motion a relationship between said oxygen exchange and the amount of hemoglobin; and wherein said variety of parameters comprise parameters reflecting oxygen consumption as well as parameters reflecting oxygen supply to the specific sites; the apparatus body further comprising a display part for performing various types of image displays based on said variety of parameters calculated by means of said controller. a controller configured to obtain, based on light information from said living body probe, by means of time series measurements, a first two-dimensional diagram plotting on polar coordinates loci showing amounts of change in oxyhemoglobin versus amounts of change in deoxyhemoglobin, and a second two-dimensional diagram plotting on polar coordinates loci showing absolute amounts of oxyhemoglobin versus absolute amounts of deoxyhemoglobin, wherein the axes of said first and second two-dimensional diagrams are inclined by 45 degrees with regards to one another; said controller being further configured to derive from said first and second two-dimensional diagrams, a variety of parameters, based on a theory assuming the oxygen exchange between oxyhemoglobin and deoxyhemoglobin in hemodynamics to be rotational motion and establishing within the equation of rotational motion a relationship between said oxygen exchange and the amount of hemoglobin; and wherein said variety of parameters comprise parameters reflecting oxygen consumption as well as parameters reflecting oxygen supply to the specific sites; the apparatus body further comprising a display part for performing various types of image displays based on said variety of parameters calculated by means of said controller.