An operating method of a flexible display device including a display unit expandable or reducible in at least one direction; a sensing unit configured to detect an expansion or reduction of a screen of the display unit; and a control unit configured to: expand the screen of the display unit according to detecting the expansion of the screen, and display a content on the expanded screen, wherein the control unit is further configured to display a ratio indicator providing information on an enlargement ratio or a reduction ratio of the displayed content in the expansion screen, wherein the ratio indicator comprises: an enlargement/reduction guide bar representing what degree of a ratio the displayed content can be enlarged or reduced.