The purpose of the present invention is to provide a device for the optical measurement of a living body, which can obtain a measurement value that is not associated with an SD distance but is proportional to a hemodynamic change at a deep part. For the purpose of separating a signal coming from the change in hemodynamics in a deep part (brain) from a signal coming from the change in hemodynamics in skin, light irradiation sites and light detection sites are positioned in such a manner that the measurement can be achieved employing at least two SD distances. At two SD distances, the amount of change in a logarithmic value of the amount of detected light at every time point is determined employing a logarithmic value of the amount of detected light under specific conditions or at a specific time point as a starting point. A gradient value for a differential SD distance, which is a difference between the amount of change obtained by the measurement at a longer SD distance and the amount of change obtained by the measurement at a shorter SD distance, is used as a measurement amount.
