The depth profiling of O 1s energy loss in silicon oxide near the SiO 2 /Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5eV was found. This value of 3.5eV is much smaller than the SiO 2 bandgap of 9.0eV, but quite close to direct interband transition at Γ point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer.