By decreasing probing depth down to 0.41 nm, O 1s photoelectron energy loss spectra with threshold energy of 3.5 eV, which was equivalent to excitation energy required for direct interband transition at gamma point in energy band structure of Si, was successfully observed even through 1.12 nm-thick-oxide film. It was found, considering the penetration of both conduction and valence band electronic states from Si substrate into SiO2 in the analysis of thickness dependence of energy loss of O 1s photoelectron caused by the direct interband transition, that the energy level at the top of valence band of oxide surface almost coincides with that of bulk-SiO2 when the thickness of silicon oxide layer exceeds (0.26 + 0.35 =) 0.61 nm. Here, 0.26 nm is a value for the thickness of interfacial compositional transition layer and 0.35 nm is a value for the thickness of one molecular layer of SiO2. In other words, the valence band structure of bulk-SiO2 can be formed by only two atomic layers of oxygen. This implies that the electronic band structure of compositional transition layer is almost equivalent to that of bulk Si. Therefore, SiO2/Si interface exists in the oxide located effectively 0.61 nm away from nominal interface defined with atomic structure.