A charged-particle beam instrument (such as a transmission electron microscope) which facilitates modifying the diameters of aperture stops installed above and below (on the beam entrance and exit sides) the specimen chamber and exchanging the aperture stops. The instrument has bottom polepieces forming the specimen chamber, aperture stops each having plural holes, pushing mechanisms for pushing the aperture stops against the polepieces and supporting the stops, and stop drive mechanisms for sliding the aperture stops in a direction perpendicular to the path of the beam in response to a manipulation performed outside the electron optical column. The aperture stops are made of a metal foil or sheet and provide a cover over the opening of at least one beam passage hole in the polepieces that faces into the specimen chamber.