In this article, we explain the contrast-formation mechanisms of the scanning-electron microscope for insulating samples under a nonpenetrating irradiation condition. Our fundamental point is that the detected secondary-electron signal current is only modified by the surface potential distribution. We propose a semiempirical model to explain the image contrast caused by excess negative-charge buildup on the surface of the insulating samples. The model treats the redistribution of secondary electrons and the contribution of tertiary electrons. The image-contrast formation mechanism of two typical examples is explained. (c) 2005 American Vacuum Society.