A flexible display device (200) is provided. The flexible display device (200) includes a display unit (250) including an expandable or reducible screen, a sensing unit (210) to detect a size of the screen, and a control unit (290) coupled with the display unit (250) and the sensing unit (210), the control unit (290) 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.