A foldable display device can include first and second casings for respectively supporting a first area and a second area of a display panel. The first and second casings are coupled to a hinge. The foldable display device can further include a slidable support which has a first end movably connected to the first casing, and a second end fixed to the second casing, and a slidable plate attached to the display panel and movably connected to the second casing. The slidable support supports a foldable area of the display panel between the first area and the second area of the display panel during an unfolding operation and an out-folding operation. Thus, the foldable display device performs both the in-folding operation and the out-folding operation.