A scanning electron microscope capable of modifying the focal position of a condenser lens with high speed and high reproducibility in order that low-magnification images are obtained at large depths of focus and that high-magnification images are obtained at high resolution. The microscope has a specimen-holding portion, an electron beam source, a condenser lens for converging the electron beam, an objective lens for focusing the converged beam into a very small spot onto a specimen, scan coils, a detector for detecting a specimen signal emanating from the specimen, and a display portion for displaying the detected specimen signal as an image. An axisymmetric electrode is disposed within the magnetic field produced by the condenser lens. A voltage is applied to the electrode.