PURPOSE: To make an image at the center part of an observation point clear and good in quality, and to remove the flitter on the entire screen by scanning electron beams helically on a sample and screen.

CONSTITUTION: Electron beam 2 from electron-beam generator 1 is focused on sample 4 by lens 3 and deflected perpendicularly by methods 5X and 5Y to scan on sample 4, and the secondary electron beam from the sample 4 is detected 6 and then supplied to the cathode of cathode-ray tube 9 by way of amplifier 7 and signal processing circuit 8. signals from generators 10X and 10Y for sine and cosine waves are modulated 12X and 12Y by the signal of saw-tooth wave genertor 11 rspectively, and both modulation outputs are supplied to deflecting methods 5X and 5Y, and 9X and 9Y, thereby achieving the helical scanning on the sample 4 and the screen of tube 9 with its speed made slow at the center and fast at its circumference.