18. The portable device of claim 11 , wherein the controller receives an input of adding a certain conversation partner through the input unit if a request for an addition of the certain conversation partner is detected through the input unit, extracts a dialog of the certain conversation partner to be added, and controls the display unit to display the extracted dialog of the certain conversation partner to be added on the sub-chatting window.
19. The portable device of claim 18 , wherein the request for the addition of the certain conversation partner corresponds to an input of dragging and dropping the certain conversation partner to be added in the sub-chatting window.