Adsorptions of monovalent atoms (noble and alkali metals) of submonolayer coverages (0.1-0.2 ML) on the Si(111)-root 3 x root 3-Ag surface commonly induced similar root 21 x root 21 superstructures, all of which exhibited high electrical conductances. Common processes seem to work among these adsorbates in such phenomena; the valence electrons of the adsorbates are transferred to the substrate surface-state bands (carrier doping). On the contrary, adsorption of C-60 molecules on the root 3 x root 3-Ag surface reduced the conductance, presumably because conduction electrons in the surface-state band of the substrate are transferred to the molecules due to their strong electronegativity (acceptor-type adsorbates).