PURPOSE: To protect a parallelism detector against damage due to continuous exposure to a strong zero-cross peak by attenuating an electron beam incident on the detector variably.

CONSTITUTION: An electron beam deflector 12 is disposed between an electronic parallelism detector 15 and a unit for dispersing an electron beam 11 into energy spectrum. When power is fed to the deflector 12, an electron energy loss spectrum is altered alternately for the active area of a detector 15 and the spectrum is deviated therefrom. More specifically, variable attenuation of the spectrum can be attained by varying the ratio of time when the spectrum is incident on a detection area to the time when the spectrum is deviated. When teak high electron loss is examined, an attenuator is generally turned off and the spectrum is held on a consecutive detector 15. When the profile of a strong zero-cross peak is examined, the attenuator is turned on and the spectrum is deviated from the detector 15 substantially.