To provide an electron gun capable of obtaining a large electron beam current at a low accelerating voltage.

This electron gun is provided with a cathode CD, Wehnelt electrodes WE, an open anode AD, current monitors IDT1, IDT2, a position monitor PDT, a computing device CL, a variable current type power source VIG and an electromagnet EMG. The magnitude of the force of a component in the direction vertical to the center axis 10 of the electron beam among forces to be applied to the electron from a magnetic field and the magnitude of the force of a component in the direction vertical to the center axis 10 of the electron beam among forces to be applied to the electron from an electric field become equal to each other at a point of a distance Zx from the surface of the cathode CD along the center axis 10 of the electron beam. This point, at which both the forces balance, exists between the cathode surface and the anode along the center axis 10 of the electron beam.