In a previous study, we developed a novel fingerprint optical sensor that can detect fingerprint patterns within the skin of a finger without considering the effect of surface condition, and the illumination position of our sensor was optimized experimentally. When a light source irradiated the root of a nail, the output image was bright without saturation. Therefore, the fingerprint image was clear, and a large area of the fingerprint pattern was clear. The aim of the present study was to clarify by experiments the mechanism underlying these results, and we hypothesized that light is scattered throughout the bone and less scattered in the muscle. Thus, the output image is strongly shaded when the nail-side position is irradiated with light. To test the hypothesis and clarify the shading mechanism, we constructed a simple simulation model that consisted of only bone and muscle and used the Monte Carlo method to simulate optical scattering in the tissue. The simulation results were in good agreement with the experimental results, which, in turn, proved our hypothesis. This study contributes to improving our fingerprint sensor, for example, by improving verification performance for use in a wide range of applications. Furthermore, our study contributes to the development of various optical sensors for use with biological tissues.