We performed time-resolved neutron reflectivity measurement for stacks of Ag 500 Å/Ge25S75 1500 Å/Si substrate and Ge33S67 1500 Å/Ag 500 Å/Si substrate to clarify silver photodiffusion process into Ge-chalcogenide layer. For Ag 500 Å/Ge25S75 1500 Å/Si substrate stack, it was found that the silver layer dissolves into Ge-chalcogenide layer within 2 min by the light exposure, and Ag-doped reaction layer forms. However, two-layer structure with thicknesses of 800 and 1100 Å was established by a prolonged light exposure for 70 min and it did not change to form one homogeneous layer. For Ge33S671500 Å/Ag 500 Å/Si substrate stack, silver rapidly dissolves into Ge33S67layer leaving a thin silver layer in the first 2 min, and then, silver slowly dissolves from the silver layer as the next reaction process. At approximately 25 min light exposure, an anomalous decrease in the neutron reflectivity, suggesting a formation of macroscopic surface roughness, was observed.