PURPOSE: To improve the voltage measurement accuracy on the surface of a sample by providing, inside respective sample chambers, detectors that detect the secondary electron and reflected electron emitted from the surface of the sample, and a circuit that differentially detects and amplifies both output signals.

CONSTITUTION: A Toneray and Everhard type secondary electron detector 4 is arranged obliquely above a sample stand 2 on which a sample 3 is loaded, while a reflected electron detector 5, for example, a semiconductor type detector that uses a silicon with a shallow pn junction is provided around the exist for the primary electron of a pole piece 1. Each detection signal of these reflected electron detector 5 and secondary electron detector 4 is amplified using head amplifiers 6 and 7 and the output signals of two head amplifiers 6 and 7 are guided to a differential amplifier 8 as the differential input. The output is amplified by a video amplifier 9, etc. and is guided to a voltage image display system at the rear step. As a result, the amount of the secondary electron signal can be kept constant and the voltage measurement accuracy on the surface of the sample can be improved.