A main control system 34 prepares, in advance, a voltage map showing the amount of focus deviation of a secondary electron beam B2 at a detection surface of an electron beam detector 30 corresponding with the amount of charge-up generated on a sample 4 upon irradiation with a primary electron beam B1, and stores this voltage map in a storage device 43. During an observation, the main control system 34 reads the voltage map stored in the storage device 43 and corrects the focal position of the secondary electron beam B2 by controlling either the voltage applied to the secondary optical system 20 or the voltage applied to the sample 4. As a result, focal position deviations resulting from charge-up generated on the sample being observed can be corrected without causing inconvenience to an operator.