PURPOSE: To obtain a great specimen current of high luminance and long life by making an improvement to the shape of a monocrystalline LaB6 cathode.

CONSTITUTION: An electron gun 1 consists of a cathode 11 of a monocrystalline LaB6 whose tip is sharpened, a heater 12 adapted to heat the cathode 11, an anode 13, and a Wehnelt 14. The electron rays from the electron gun 1 is focused into a crossover at the point a. The image of the crossover is contracted by condenser lenses 2 and 3 and then projected onto the specimen 5 through an object lens 4. The cathode curvature radius at the concave tip of the cathode 11 is 20W40μm.