A hemisphere accelerating electrode has a double structure composed of an inner accelerating electrode and an outer accelerating electrode. The inner accelerating electrode has an inner introducing inlet and an inner opening, and the outer accelerating electrode has an outer introducing inlet and an outer opening. The aperture angle of the inner introducing inlet is preferably larger than that of the outer introducing inlet by 0.1-5 degrees. Then, the aperture angle of the inner opening is preferably larger than that of the outer opening by 0.1-5 degrees. Moreover, scattered electron detectors have correcting electrodes, respectively, and are arranged in the shifted directions from the introducing direction of electrons by 100-140 degrees.