A plurality of panels include: a display panel; and a protective panel that is disposed outside the display panel, and includes a transparent substrate and an outer hard coating layer and an inner hard coating layer that are respectively formed on an outer surface and an inner surface of the transparent substrate, and each have a hardness greater than a hardness of the transparent substrate. A thickness of the outer hard coating layer is greater than a thickness of the inner hard coating layer, and a stress neutral surface of the flexible display device is configured so that a compressive stress is applied to the inner hard coating layer when the flexible display device is bent.