In a magnetic field immersion type electron gun for controlling an electron beam emitted by an electron gun (51) with the use of an electric lens (56) and a magnetic field lens formed by permanent magnets (57, 58) of a coaxial ion pump (53), the ion pump magnets are a pair of cylindrical permanent magnets (57, 58) disposed coaxially with an optical axis (52) of the electron gun (51) in such a way as to sandwich a cylindrical ion pump anode (61) of the coaxial ion pump; the two permanent magnets are magnetized in a mutually opposing direction; a hollow cylindrical yoke (60) is disposed also coaxially with the optical axis (52) in such a way as to enclose the two permanent magnets (57, 58) within a hollow portion thereof; and the yoke (60) is formed with an annular yoke gap (63) in a radially inner circumferential surface of the yoke (60) to leak out a magnetic flux flowing through the yoke toward the optical axis. In the above-mentioned construction, the magnetic field lens can be formed efficiently with the use of the magnetic field generated by the permanent magnets for constituting the coaxial ion pump, and further the formed magnetic field lens can be superimposed upon the electron gun. Therefore, an electric field immersion type electron gun of high performance can be obtained, and further the electron gun chamber can be efficiently evacuated in the vicinity of the cathode tip of the electron gun.