An optical measurement instrument for a living body to measure dynamic change of blood of a brain includes light incident units for irradiating light on a head of a subject, a light detector for detecting a light irradiated from the light incident units and reflected within the subject, a signal processing unit for calculating a spacious distribution of signals from the subject to be measured from a signal detected by the light detector, and a display for displaying the spacious distribution of signals from the subject to be measured. Measurement points are arranged about a middle of the light incident units and the light detector, and the light incident units and the light detector are arranged such that the measurement points are arranged to form a lattice whose circumference is square.