an electron gun includes an electron source configured to emit electrons . the electron source includes an electron emission region configured to emit the electrons and an electron emission restrictive region configured to restrict emission of the electrons . the electron emission restrictive region is located on a side surface of the electron source except an electron emission surface on a tip of the electron source and is covered with a different material from the electron source . the electron gun emits thermal field - emitted electrons by applying an electric field to the tip while maintaining a sufficiently low temperature to avoid sublimation of a material of the electron source . the material of the electron source may be lanthanum hexaboride or cerium hexaboride . the electron emission restrictive region may be covered with carbon .