A flexible display device is provided. The flexible display device includes a display unit including an expandable or reducible screen, a sensing unit to detect a size of the screen, and a control unit coupled with the display unit and the sensing unit, the control unit is further capable of: displaying a first execution window of a first application on the screen corresponding to a first distance; and executing a second application if the size of the screen is expanded by an area corresponding to a predetermined distance in a first direction after the size of the screen is reduced by the area corresponding to the predetermined distance in a second direction.