An ion source apparatus comprising a bulk type thermionic cathode, which, when heated, emits thermoelectrons; an anode for causing a gas discharge in cooperation with the cathode thereby producing plasma; a grid electrode for extracting ions out of the plasma; a magnetic member for confining the plasma within a prescribed region; and a ferromagnetic body which surrounds the thermionic cathode. The ferromagnetic body removes the lines of magnetic force produced by magnets from the neighborhood of the surface of the thermionic cathode. As a result, thermoelectrons are freely emitted from the thermionic cathode, thereby ensuring a stable gas discharge.