The secondary electron (SE) images were recorded for Si wafers with and without SiO₂ thin (10 nm-50 nm) films by a scanning electron microscope (SEM) with changing primary electron energy from 2.5 keV down to 0.2 keV during one frame scan. Brightness of the SE image increased up to critical values and then decreased with decreasing primary electron energy for the specimens with SiO₂ films. The critical voltage is higher for specimen with thicker film. These results are discussed by means of the oxide film thickness, the penetration depth of incident electrons, and the surface potential caused by charging. [DOI: 10.1380/ejssnt.2008.35]