An image generating methodology for displaying brain activation areas with improved accuracy in a living body light measurement system for generating an image of the changes in blood volume of the brain. A spatial intensity distribution of changes in blood volume can be generated by detecting brain activation in many sampling points, and a spatial interpolation process is executed on this data. A threshold process is executed on the distribution to extract the distribution of coordinates having a signal intensity of at least the predetermined threshold. The displacement between the brain activation area and the maximal location of the distribution is stored in a recording unit of the system in order to compensate for the distribution extracted by the threshold process. Accordingly, brain activation areas can be estimated with greater location accuracy, and the diagnosis and medial treatment for brain diseases can likewise be executed with greater accuracy.