To provide a small electron gun capable of keeping a high vacuum pressure used for an electron microscope and an electron-beam drawing apparatus. An electron gun constituted by a nonevaporative getter pump, a heater, a filament, and an electron-source positioning mechanism is provided with an opening for rough exhausting and its automatically opening/closing valve, and means for ionizing and decomposing an inert gas or a compound gas for the nonevaporative getter pump. It is possible to keep a high vacuum pressure of 10−10 Torr without requiring an ion pump by using a small electron gun having a height and a width of approximately 15 cm while emitting electrons from the electron gun.