The lateral diffusion of the Ag deped in the chalcogenide glasses (AS₂S₃, AS₂Se₃) was studied, relating with the inherent resolving power of the Ag-chalcogenide sensors. During the photodoping of Ag into the chalcogenide glass, vertical diffusion was smeared simultaniously the boundary zone between the Ag doped area and the undoped one due to the lateral diffusion of Ag. The lateral diffusion began to proceed rapidly when the chalcogenide layer was saturated with Ag doped. The lateral diffusion was measured spectrophotometrically by using a microspectrophotometer. Thermal diffusion depended greatly on the chemical composition of the glasses and was conspicuously observable in the glasses consisting of Se or Te at room temperature. The images formed on AS₂S₃ glass were stable, since the thermal diffusion of Ag in the glass was negligible small. MTF (Modulation Transfer Function) curves were obtained an Ag-AS₂S₃ sensor in various amount of Ag doped in the glass. It was confirmed from the MTF that the sensor could kept inherently high resolving power, though the lateral diffusion caused the degradation of resolution as a large amount of Ag was doped.