PURPOSE: To easily judge whether the result of an automatic length measurement is correct or not when the result is deviated from a tolerance by a method wherein the differentiated waveform of a line profile signal waveform is displayed on a screen.

CONSTITUTION: An electron beam 2 from an electron gun 1 is narrowed by an objective lens 6, and a sample 7 is irradiated with the beam. The lens 6 is excited by an objective-lens power supply 11. A deflecting-signal generator 14 is instructed by a computer 21, it scans the beam 2 on the sample 7 two- dimensionally via a deflecting coil 3. The computer 21 reads the whole or a part of image data in an image memory 16, and it superposes and displays the scanned image of an object to be observed and the differentiated signal waveform expressing the change rate of a line profile signal waveform on the screen of an image-displaying CRT 19. Then, since many extreme values exist in the differentiated waveform and the largest extreme value of peak values does not correspond to a normal edge, a cause that the result of a length measurement is outside a tolerance can be recognized as caused by a length- measuring error.