A flexible display device according to example embodiments includes a rollable display panel, a housing accommodating the rollable display panel in a rolled state and including an opening portion through which the rollable display panel is pulled out, an optical sensor disposed at the opening portion and configured to detect luminance of the rollable display panel while the rollable display panel is rolled or unrolled, a controller configured to compensate image data based on detection data generated by the optical sensor, and a display panel driver configured to control a display of rollable display panel.