PURPOSE: To simultaneously control the primary electron and the secondary electron in an optimum state, and to automatically analyse in an optimum state by a method wherein the charged potential on the cover surface of a sample is computed by a charged potential measuring means, the optimum buffer voltage is set by an analyser operational condition optimizing means and an objective lens is focused by computer control.

CONSTITUTION: Analytical voltage, with which the curvature of energy analytical curved line of the secondary electron negatively becomes the maximum, is computed by a charged potential measuring means 1, and the charged potential on the cover surface of a sample SPL is computed by obtaining the difference between the above- mentioned analytical voltage and the analytical voltage obtained when there is no electrification. Using an analyzer operational condition optimizing means 3, the measured value of electrification voltage is regarded as the medium of analytical voltage value, this value is added to the optimum buffer voltage value, which is experimentally computed in advance, of the case of non-electrification sample, and optimum buffer voltage is set. The focusing of an objective lens is conducted by the control of a computer using a focus adjusting means 5. As a result, the operational condition of an analyzer can always be optimized irrespective of the condition of electrification of the sample surface.