An electron beam apparatus has the electron optical column for releasing an electron beam from the front-end portion after the beam is emitted from an electron beam source located on a rear-end portion of the column, a specimen chamber connected to a front-end portion of the column, and an aperture member withdrawably disposed in the front-end portion of the column within the specimen chamber. The apparatus further includes a rotating mechanism for rotating the aperture member along a given plane lying along the direction of a path of the beam. Thus, the aperture member can be attached and detached to and from the front-end portion of the column.