The anomalous diffusion process of Ag into a GeS2 film induced by the illumination of light with the photon energy near or larger than the optical gap of a GeS2 film was examined in-situ by the spectroscopic ellipsometry in the wavelength region between 450 and 850 nm. The physical parameters such as the thickness and the refractive indices n and K for each layer were derived from the ellipsometric parameters measured at respective wavelength using the simulation assuming a three layers (Ag, Ag: GeS2 and GeS2 layers) model. Two steps of doping process called the first and second stage were derived through the analyses of the optical constants, which were different from the results for As2S3 films. Especially, the first stage progressed very fast in GeS2 films in the three layers model. The in-situ observations using an atomic force microscope were also carried out to understand the doping process through the change of the surface morphology during doping of Ag and the results which supported the ellipsometric behaviors were obtained. Other simulation model assuming optical constants considering effective medium approximation for the doped layer during doping was discussed for the analysis of ellipsometric data and the difference of the doping behavior between two models was discussed. (C) 2013 Elsevier B.V. All rights reserved.