PURPOSE: To do accurate correction of a linear function equation to improve the measurement accuracy by sequentially updating a beam-off level Se-off with the output of a secondary electron detector.

CONSTITUTION: A measuring pad 2a is provided in a vacuum chamber 1, voltage to be measured is applied from the outside to the pad 2a, and an electron beam BP is applied to the pad 2a while an energy analyzer 5 is placed close to the pad 2a. Based on a predetermined relational equation representing a corresponding relation between analysis voltage Vr applied to the energy analyzer 5 and secondary electrons BR from the pad 2a taken out based on the analysis voltage Vr, voltage to be measured which is led to the pad 2a is measured.