A method for controlling a display device comprising a touch screen is provided. The control method displaying, on the touch screen, a plurality of windows that do not overlap one another and that respectively correspond to executing, displaying a center button disposed at an intersection of a plurality of dividing lines that distinguishes the plurality of windows, receiving an input of a window size change command for changing a size of at least one of the plurality of windows, changing the size of at least one of the plurality of windows in response to the window size change command, and discontinuing the displaying of less than all of the plurality of windows and enlarging a size of at least one remaining window.