A transmission target of a vacuum container is operable to have a ground potential and an electro-optical system is floated at a positive potential in the vacuum container. An electron beam, which is converged by means of the electro-optical system, is decelerated immediately before the electron beam is incident to the transmission target. The electron beam has energy that is several times of the final set value until the electron beam passes through the electro-optical system, and a divergence action exerted by a spatial electric charge effect is reduced. Color aberration of the electro-optical system is proportional to energy of the electron beam. Thus, if the electron beam is decelerated after the electron beam has passed through the electro-optical system, aberration is reduced in proportion to the degree of deceleration, making it possible to concurrently reduce a focus size.