An electrode unit of an electron gun for a cathode ray tube including an outer rim electrode having a large-diameter electron beam passing hole through which three electron beams pass, and an inner electrode installed within the outer rim electrode and having a central electron beam passing hole disposed at its center and side electron beam passing holes disposed at opposite sides of the central electron beam passing hole. The side electron beam passing holes each have first and second curved sides along the sides closest to the central electron beam passing hole and opposite those sides, and linear portions connecting the first and second curved portions. The vertical and horizontal dimensions of the side electron beam passing holes are equal to each other.