In a scanning electron microscope, an optimum scanning method for reducing the amount of deflection of a primary electron beam and secondary electrons is determined to acquire stable images. An energy filter is used to discriminate between energy levels. The change in yield of obtained electrons is used to measure the variation in specimen potential. The time constant of charging created during electron beam irradiation is extracted. The scanning method is optimized based on the extracted time constant to reduce the distortion and magnification variation that appear in a SEM image.