Silver photodiffusion into amorphous chalcogenides is the photoinduced phenomenon, in which mobile Ag ions are injected into semiconductor films, and is applicable to nonvolatile memory devices. To understand the role of light illumination in the silver diffusion into a chalcogenide layer, the excitation light energy dependence of silver photodiffusion in Ag/a‐Ge20S80/Si substrate stacks is investigated by neutron reflectivity, X‐ray reflectivity, and X‐ray diffraction. The measurements reveal that there is an energy threshold to induce silver photodiffusion, which corresponds to the optical gap of amorphous Ge20S80. The excitation of the lone‐pair electrons to the antibonding states by the light illumination with greater energy than the optical gap leads to the bond breaking, and the Ge–S network structure is reorganized involving Ag ions.