1. An emotional state determination method of measuring at least one of oxy-hemoglobin densities and deoxy-hemoglobin densities in blood of a plurality of measurement regions of a human brain cortex of a subject in time series, respectively, using a near-infrared spectroscopy, and determining an emotional state of the subject based on at least one of time-variable change data on the measured oxy-hemoglobin densities and time-variable change data on the measured deoxy-hemoglobin densities, the method comprising: in respect of plural sets of the time-variable change data, each of the sets comprising two of the time-variable change data which are selected by permutations and combinations from among the at least one of the time-variable change data on the oxy-hemoglobin densities and the time-variable change data on the deoxy-hemoglobin densities measured for the respective measurement regions of the subject under each of plural types of conditions that influence emotion of the subject, a reference data collection step of computing, for each unit time, cross-correlation coefficients of the plural sets of the time-variable change data, and collecting a plurality of time-variable change patterns of the computed cross-correlation coefficients with respect to the plural sets of the time-variable change data as a plurality of reference data; a judging condition determination step of determining a judging condition necessary for determining the emotional state of the subject by a predetermined determination method, from the plurality of reference data obtained in the reference data collection step; in respect of plural sets of the time-variable change data, each of the sets comprising two of the time-variable change data which are selected by the permutations and combinations from among the at least one of the time-variable change data on the oxy-hemoglobin densities and the time-variable change data on the deoxy-hemoglobin densities measured for the respective measurement regions of the subject under a predetermined condition, an evaluation data collection step of computing, for the each unit time, cross-correlation coefficients of the plural sets of the time-variable change data and collecting a plurality of time-variable change patterns of the computed cross-correlation coefficients with respect to the plurality sets of the time-variable change data as a plurality of evaluation data; and a determination step of inputting the judging condition determined in the condition determination step and the plurality of evaluation data, and quantitatively determining the emotional state of the subject by the predetermined determination method.