To solve a problem in a conventional SEM, wherein the focus cannot be adjusted in a charged sample, or magnification cannot be correctly calculated, whereby a correct pattern dimension cannot be measured.

A retarding voltage is swept between a retarding voltage, with a primary electron beam not reaching a sample and a retarding voltage with the primary electron beam reaching the sample; the output of a secondary electron detector is detected, to acquire the retarding voltage and characteristics of the output of the secondary electron detector; and the potential of a sample surface is obtained based on the shift of the retarding voltage and the characteristics of the output of the secondary electron detector.