To provide a method for plating Ag-Sn onto a copper or copper alloy plate, and to provide an Ag-Sn-plated copper or copper alloy plate which is produced by the method and excellent in sulfurization resistance and resistance to ultraviolet light, and has high total reflectivity and superior reflection directivity of light.

The method for plating Ag-Sn onto a copper or copper alloy plate includes: forming a glossy Sn-plated layer on the surface of the copper or copper alloy plate; then removing the oxide film formed on the surface of the Sn-plated film by an electrolytic treatment or immersion treatment in a weak alkaline solution; electroless plating or electroplating Ag; keeping the resulting copper or copper alloy plate at 40-60°C for 30-300 s to form a completely alloyed Ag₃Sn film; and further immersing the Ag₃Sn film into a phosphoric acid-based oxidation treatment agent having a temperature of 20-30°C for 30-120 s to form an Sn oxide film on the Ag₃Sn film.