PURPOSE: To prevent the point of impact of an electron beam from being fluctuated during measurement by providing an aperture which has a large opening window corresponding to the swinging with of an electron beam between an objective lens and a device to be measured.

CONSTITUTION: An electron beam produced from an electron beam generator is converged by an objective lens 1 and is applied to a device 2 to be measured. An aperture 3 is provided between the objective lens 1 and the device to be measured. The aperture 3 is constituted of a first plate-like aperture 4 and a second plate-like aperture 5 which are arranged close to each other. The aperture 4 and the aperture 5 respectively are relatively displaced in the direction vertical to the direction of the passage of the electron beam. The size of an opening window 6 can be changed by an amount of relative displacement. The size of the opening window 6 can be set to nearly same size as the swinging width of a primary electron beam. Thereby, electromagnetic waves from the region in which electron charges exist is effectively shielded by the aperture 3.