Abstract: A sample in a sample chamber is irradiated for scanning with an electron beam emitted through a pressure limiting aperture. Secondary electrons reflected from the sample are gas amplified by gas in the sample chamber. A member defining limiting aperture serves also as an electrode, and another electrode is provided around the member. The member can move along a passage for the electron beam emitted from an electron gun by means of, for example a motor, a pinion spindle, a pinion, a rack, and a connecting member. The pressure limiting aperture is positioned close to the sample when a contact hole is observed and is positioned away from the specimen when the surface of the sample is observed.