A cathode ray tube electron gun having an electron beam-generating source and a main lens system for focusing electron beams emitted from said electron beam-generating source, wherein the main lens system comprises at least first, second, third and fourth focusing electrodes arranged coaxially and in the order mentioned an counted from said electron beam-generating source; the first and third focusing electrodes are impressed with substantially the same potential; and the fourth focusing electrode is supplied with a prescribed potential higher than those which are applied to said first and third focusing electrodes.