Changes in the optical transmission spectra resulting from Ag photodoping in thin Ge30S70 film were analyzed on the basis of a uniform distribution of dissolved Ag. This study suggests that photodoping rate is quantitatively evaluated from time dependence of 1n[ T(t)/T(O) ] obtained from the spectra of thin photodoping sample, where T(t) is the transmission after exposure to light for time t . The relations between initial photodoping rate and photon energy of irradiated light were investigated for the irradiation of light from Ag-layer side and Ge30S70-layer side, respectively. The difference in the initial photodoping rate for the irradiated sides was interpreted on the basis of the intensities of light reached to the Ag-doped layer through the Ag layer and the Ge30S70 layer. The following results were obtained: (1) for the exposure to light with low photon energy, the light absorbed in the Ag-doped layer causes the photodoping; (2) for the exposure to light with high photon energy, the light absorbed in the chalcogenide layer causes mainly the photodoping; (3) our experimental results suggest that the light absorbed in the Ag layer is not effective in the photodoping.