Optical-CT imaging, is a desired advanced technology for non-invasive measurements of the oxygenation state by optical methods. The difficulty resulting from the strong scattering of light by living tissues has hindered its development. Fundamental investigations have been conducted to realize optical-CT imaging by using time-resolved spectroscopy. Chance et al. have reported the usefulness of time-resolved spectroscopy to measure the oxygenation state in brain, stating that the slopes of the decay curves obtained by pulsed-light incidence can provide the absorbance change. Delpy et al., Wilson et al., and Nomura et al. have also used time-resolved spectroscopy to measure optical properties of tissues or to analyze fundamental phenomena of light-tissue interaction. Patterson et al. analyzed the behavior of light in tissues by using the time dependent diffusion approximation, while Delpy et al. and Hasegawa et al. applied the Monte Carlo method to simulate the time-resolved transmittance. The results of Chance et al., Nomura et al. and Hasegawa et al. indicate that the Beer-Lambert law holds, even in strongly scattering media, when the path-length of the scattered light is microscopically counted. This suggests that optical-CT imaging might be possible if the path-length of the scattered light is traced by time-resolved spectroscopy.