Using low-temperature scanning tunneling microscopy/spectroscopy we have developed a method for measuring electrostatic potential in high spatial and energy resolutions, and performed a real-space observation of the potential screened by two-dimensional surface electrons around step edges, where extra charges are localized, on the Si(1 1 1)root 3 x root 3-Ag surface. In the potential images, characteristic decay and the Friedel oscillation were clearly observed around the charges. (C) 2009 Elsevier B.V. All rights reserved.