An input instrument for a living body, using an optical measurement system for the living body, includes first and second light incident units arranged on a head of a living body, and first and second light detectors which are paired with the first and the second light incident units respectively, and which are provided to collect light which passes through the living body based on the light irradiated onto the head of the living body by the light incident means. Measurement signals are obtained by measuring a plurality of regions of the living body by the first and the second light incident units, the first and the second light detectors, and characteristic parameters of the signals of an arbitrary time interval of the measured signals are calculated.