1. A scanning electron microscope comprising: an electron beam source; a lens system for converging a primary electron beam emitted from said electron beam source onto the surface of a sample; a scanning unit for tow-dimensionally scanning the converged primary electron beam on the surface of the sample; a detector for detecting a secondary signal generated from the sample irradiated with the primary electron beam; an amplifier for amplifying a signal detected by said detector; a rendering unit for forming a sample image based on the signal amplified by said amplifier; and a control processing unit, said control processing unit configured to create a luminance histogram of a sample image, control a current value of the primary electron beam such that the brightness of the image calculated from the luminance histogram is located substantially at the center of an overall halftone width, and control an amplifying condition for said amplifier to provide a contrast corresponding to the brightness of the image.