X-ray diffraction and reflectivity as well as transmittance on Ge-As-S bulk and thin films were performed. It was observed photodarkening in Ge-As-S thin films with band gap light illumination from an argon laser of 514.5 nm wavelength and shifting the optical transmittance edge to shorter wavelength, the magnitude of which increases with the increase of the intensity of illumination light. In the Ge-As-S bulk, however, it was not observed photodarkening. Photodarkening in Ge-As-S system was discussed.