In an environmental scanning electron microscope in which differential pumping for maintaining the pressure ratio between an electron optical system and a specimen chamber at a predetermined value is effected and a probe electric current is conditioned to meet a predetermined or more value so as to permit observation of uncooked food and moist specimens in low vacuum, there are provided three stages of objective apertures used as apertures for an objective lens for an electron beam in the electron optical system and used also as orifices for differential pumping for maintaining the pressure ratio between the electron optical system and the specimen chamber at a predetermined value. Then, a deflection fulcrum of the electron beam in the electron optical system is set at a mid stage of the three-stage objective aperture.