To realize highly reliable thin oxide with low leakage current, Si surface topography should be atomically controlled. The authors reported previously that SiO2/Si interface topography with wide step/terrace structures was roughened during thermal oxidation for comparatively thick SiO2 film at higher temperatures. On the other hand, Watanabe et al. (1998, 2000) and Miyata et al. (1998) reported a layer-by-layer oxidation at the SiO2/Si interface using scanning reflection electron microscopy (SREM) for relatively thin SiO2 film. The layer-by-layer oxidation means that the interface topography is preserved during oxidation. It apparently seems that those two phenomena are inconsistent with each other. In this study, roughness at the SiO2/Si interface was atomically investigated using AFM in an intermediated SiO2 thickness.