The modification of the surface electronic structure by an adsorbate is measured quantitatively with scanning tunneling microscopy and spectroscopy for the first time. The standing wave of the Cu surface-state electrons is utilized to probe the subtle change in the electronic structure on the Xe-covered Cu(111). The observed Fermi wavelength on the Xe-covered surface is longer by 15% than on the bare Cu surface. The change upon Xe adsorption is explained with the observed modified dispersion of the Cu surface state; upward shift by $(130\ifmmode\pm\else\textpm\fi{}20)\mathrm{meV}$ with almost same effective mass.