1. An apparatus for measuring a brain local activity comprising: a plurality of sensors mounted on a head of a subject for measuring scalp potentials of the subject; and a computing unit dividing, for each sensor, a predetermined frequency bandwidth wider than a frequency bandwidth of alpha waves of the scalp potentials outputted from the sensors into a predetermined number of frequency banks each having a fixed frequency bandwidth, dividing, for each sensor, data of each divided frequency bank into segments of a predetermined duration on a time axis, determining, for each sensor, a Z-score of the subject from a first mean value, for each frequency bank, of normalized power variances determined for the segments and a second mean value, for each frequency bank, of normalized power variances predetermined in the same manner as the first mean value for a predetermined normal person group and a standard deviation of the normalized power variances in the group, and mapping on a brain surface for each sensor a mean value of the Z-scores determined over all of the frequency banks.