To study optical properties of Ag-Au alloys as the simplest homogeneous diordered alloys, the dependence of the reflectivity of Ag-Au alloys on their composition has been measured with the wave lengths ranging from 700m${mu}$ to 900m${mu}$. Measurements were made on mechanically polished bulk samples and the experimental results are discussed in terms of the Drude-Zender theory. The distributions on the reflectivity of various wavelengths are also discussed.