Diffuse Optical Tomography (DOT) is one of the forthcoming medical technologies, and it is expected to have great development especially in brain science. DOT is considered an inverse problem for the radiative transport equation, and we try to show background of mathematical modeling for propagation of light in biological tissues through numerical and mathematical analysis. This work is a joint work with Prof. Hiroshi Fujiwara, Dr. Daiki Tanaka and some other graduate students.