Tunneling spectroscopy of ultrathin (1.5 nm thick) SiO2on degenerate Si structures and of hydrogen‐terminated Si surfaces is studied with scanning tunneling microscopy (STM) in an air ambient. Two kinds of tunneling spectra, i.e., normal‐site and defect‐site spectra, are observed for the oxide samples depending on measuring sites, while only the normal‐site spectra are observed for H‐terminated surfaces. The normal‐site spectra strongly depend on dopant types and reflect bulk band structures of Si. The defect‐site spectra show negative differential resistance (NDR) and the defect sites are identified on STM images as depressed areas. The origin of the NDR is ascribed to resonant tunneling through localized defects in the oxide.