Apparatus for high-angle annular dark-field (HAADF) imaging without limiting the maximum tolerable pressure of gas introduced into the specimen chamber (7). The apparatus has an electron gun (2), a specimen chamber in which a specimen (5) is set, a gas cylinder (37) for supplying environmental gas to around the specimen surface through both a gas flow rate controller (39) and a gas nozzle (40), a vacuum pump (36) for evacuating the inside of the specimen chamber, an objective lens including upper and lower polepieces (8',9'), a detector (21,22,14,15) for detecting electrons transmitted through the specimen, a display device (19) for displaying a transmission image of the specimen based on the output signal from the detector, orifice plates (50) having minute holes (A-E), holders (51) supporting the orifice plates, a drive mechanism (52) for driving the holders, and a motion controller (53). The specimen is set between the upper and lower polepieces. The beam from the electron gun is transmitted through the objective lens. The orifice plates can be moved in a direction crossing the optical axis of the beam on the upper and lower surfaces of the upper and lower polepieces of the objective lens.