An electron gun 901 capable of suppressing an uneven temperature distribution at an extraction electrode and a length-measuring SEM 900 are provided. The electron gun 901 is equipped with: a charged particle source 1; an extraction electrode 3 for extracting charged particles from the charged particle source 1 and allowing some of the charged particles to pass while blocking some other charged particles; and an auxiliary structure 5 disposed in contact with the extraction electrode 3. The length-measuring SEM 900 is equipped with the electron gun 901 and a computer system 920 for controlling the electron gun 901.