Provided is an electron microscope having an electron gun emitting an electron beam onto a specimen to be observed, so as to produce transmitted main electron rays, inelastic scattering electron rays and diffractive scattering rays, an objective lens converging the electron rays so as to form an image, an objective aperture for normally cutting off the diffractive scattering electron rays while allowing the transmitted main electron rays and the inelastic scattering electron rays to pass therethrough, a magnifying means for magnifying the image, an observing screen from which the magnified image is visible, a mechanism for deflecting the electron beam emitted from the electron gun before the specimen, and a means for controlling the mechanism to deflect the electron beam to a large deflecting angle so as to cut off the main transmitted electron rays and the inelastic scattering electron rays at the objective aperture while allowing the diffractive scattering electron rays to enter into the magnifying lens through the objective aperture so as to form a dark field image having high contrast, whereby the focusing is made while observing the dark field image.