An electron gun for a cathode ray tube comprises a triode including a cathode, a control electrode and an accelerating electrode, a pre-focusing electrode unit adjacent to the triode, a main lens unit including a focusing electrode and an anode for forming a main lens for focusing the electron beam toward a screen, a first focusing electrode unit having vertically-elongated electron beam passing holes and horizontally-elongated electron beam passing holes for forming a quadrupole lens, a second focusing electrode unit having vertically-elongated electron beam passing holes and horizontally-elongated electron beam passing holes for forming a quadrupole lens, and an auxiliary electrode disposed between the first focusing electrode unit and the second focusing electrode unit, to which a dynamic voltage is applied, and including vertically-elongated electron beam passing holes on electron beam incoming side thereof and horizontally-elongated electron beam passing holes on electron beam outgoing side thereof.