An electrostatic potential profile was obtained on the Si(111)√3×√3-Ag surface at a subnanometer spatial resolution. The potential was measured from an energy-level shift of electronic states on the surface. The obtained potential images reveal that the potential drops around steps and Ag adsorbates, upon which positive charges are presumably accumulated. The profiles of the reduced potentials are explained with the screening of potential due to the charges in two-dimensional electron gas (2DEG) existing on the surface. The Friedel oscillation, which results from the screening and has a period of the half Fermi wavelength of the 2DEG, was also observed in the potential images. The relative variation of the measured potential is in sub-10 meV range, demonstrating high energy resolution of the potential measurement method.