1. A method for measuring an internal property distribution, comprising: making measurement light incident successively into a measurement object from a plurality of light incidence positions onto a surface of said measurement object; detecting the measurement light, said measurement light having passed through the measurement object successively or simultaneously, at at least one light detection position out of a plurality of light detection positions on the surface of said measurement object and in a predetermined positional relation with respect to a light incidence position at which the measurement light to be measured was incident; obtaining a measured value of a predetermined parameter of said measurement light based on incident measurement light detected at each light detection position; extracting a plurality of said measured values obtained by a plurality of combinations of said light incidence position and said light detection position, said positional relation of which is relatively identical, and calculating a mean value of the measured values to obtain a reference value in the positional relation; and calculating a change amount of a predetermined internal property in each region of said measurement object divided into a plurality of regions using said plurality of measured values obtained by said plurality of combinations, and said reference value, thereby obtaining an internal property change amount distribution in the measurement object.