An electron gun cathode (104) is column shaped, and emits electrons by being heated. A holder (103), which covers the bottom and sides of the electron gun cathode, has electrical conductivity and holds the electron gun cathode, and is composed of a material that does not easily react with the electron gun cathode when in a heated state, is provided. The tip of the electron gun cathode (104) protrudes from the holder (103) so as to be exposed, and electrons are emitted from the tip toward the front by applying an electric field to the tip.