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Work function is crucially important for the characteristics of electron emission from solid surfaces. For most of elemental metals, work function is well established experimentally and theoretically. However, for alloys or compounds, work function has not been sufficiently discussed so far. Microscopic investigation is necessary to understand the origin of work function of alloys or compounds. By using a scanning tunneling microscope (STM), we can obtain atomic-scale images of the local tunneling barrier height (LBH), closely related to the microscopic work function. In a simplified one-dimensional electron-tunneling model, LBH is equal to the average of the microscopic work functions of tip and sample surfaces. Thus, atomic scale distributions of work function are derived from the LBH images. In this study, we examine Ag-adsorbed Pt(lll) surfaces, which are known to form two-dimensionally confined Ag-Pt alloy as stated in H. Roeder et al. (1993).