Discrete Fourier transform is performed on an output of each brain potential sensor, which measure a subject's brain potential, for each segment in order to obtain a discrete Fourier coefficient that has a frequency component. A mean value of squares of absolute values of Fourier coefficients is obtained. The Fourier coefficients are normalized using the mean value for obtaining a normalized power spectrum NPS；j,m. Mean values of squares of absolute values of Fourier coefficients of adjoining frequency components in all the segments is normalized using a square value of the mean values of the adjoining frequency components for obtaining a normalized power ratio NPV；j,m. Two markers sNAT；j,m and vNAT；j,m are derived from the power spectrum and power ratio for evaluating a brain function activity level.
