A method for controlling a resolution of a graphic image, comprising the steps of: (a) selecting a resolution value of the graphic image to be displayed on an external display unit, wherein the resolution value is a horizontal and vertical resolution value; (b) additionally selecting another resolution value corresponding to a valid screen of the external display unit, wherein a size of a valid screen displaying a video image is smaller than that of an actual physical screen, wherein additionally a horizontal and vertical resolution value corresponding to a valid horizontal and vertical frequency is selected so that the graphic image controlled by the resolution value selected at the step (a) can be fully displayed on a valid screen; (c) confirming a source type of the graphic image to be displayed on the external display unit and a screen mode; and (d) referring to the selected resolution values and reconfiguring the graphic image, according to a result of the confirmation, and (e) characterized by shifting a start point of the graphic image controlled by the horizontal and vertical resolution value selected at the step (b) to within a valid display period contained between horizontal sync signals and excluding part of a rear end of the graphic image from a valid display period, such that the number of pixels of the graphic image in a horizontal direction is reduced and the number of lines of the graphic image in a vertical direction is reduced, wherein the step (d) is carried out by controlling the resolution of the graphic image on the basis of the resolution value selected at the step (a) where a source type of the confirmed graphic image is a video image and a screen mode is a full screen mode, wherein the step (d) is carried out by referring to the resolution values selected at the steps (a) and (b) and controlling the resolution of the graphic image, where a source type of the confirmed graphic image is another image rather than a video or broadcast image, or is a video or broadcast image based on another screen mode rather than a full screen mode, wherein the part of the rear end of the graphic image excluded from the valid display period corresponds to an amount the start point of the graphic image is shifted to within the valid display period. (a) selecting a resolution value of the graphic image to be displayed on an external display unit, wherein the resolution value is a horizontal and vertical resolution value; (b) additionally selecting another resolution value corresponding to a valid screen of the external display unit, wherein a size of a valid screen displaying a video image is smaller than that of an actual physical screen, wherein additionally a horizontal and vertical resolution value corresponding to a valid horizontal and vertical frequency is selected so that the graphic image controlled by the resolution value selected at the step (a) can be fully displayed on a valid screen; (c) confirming a source type of the graphic image to be displayed on the external display unit and a screen mode; and (d) referring to the selected resolution values and reconfiguring the graphic image, according to a result of the confirmation, and (e) characterized by shifting a start point of the graphic image controlled by the horizontal and vertical resolution value selected at the step (b) to within a valid display period contained between horizontal sync signals and excluding part of a rear end of the graphic image from a valid display period, such that the number of pixels of the graphic image in a horizontal direction is reduced and the number of lines of the graphic image in a vertical direction is reduced,