By decreasing probing depth down to 0.41 nm, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV, that was equivalent with excitation energy required for direct interband transition at Γ point in energy band structure of Si, was observed even through a 1.12-nm-thick oxide film. It was found, by considering the penetration of electronic states from Si substrate into SiO₂ in the analysis of thickness dependence of the energy loss of O 1s photoelectrons caused by the direct interband transition, that the top of valence band of compositional transition layer is almost equivalent to that of bulk Si. In other words, the SiO₂/Si interface defined by electronic structure exists in the oxide located effectively 0.61 nm away from the nominal interface defined by chemical structure.