To provide an ion source of an ion implantation device capable of thinning the diameter of ion beam cross section in a simple method, reducing beam loss, reducing contamination on the inside of an implantation unit and insulation deterioration, and increasing effective beam density.

An ion source of an ion implantation device has a reflector electrode 4 for reflecting thermoelectrons generated in a filament 1 and a DC magnetic field apply means for applying a DC magnetic field advancing the thermoelectrons from the filament 1 to the reflector electrode 4. A DC voltage variable means 7 for varying DC voltage applying to the reflector electrode 4, and an RF power source 8 for applying high frequency AC voltage superimposing on the DC voltage are arranged, and an electric field between the filament 1 and the reflector electrode 4 is varied by applying these voltages.