The present disclosure relates to a flexible display device including a first body; a second body configured to be relatively movable with respect to the first body; a flexible display disposed on a front surface of the first body and a rear surface of the second body, and configured to vary in size of areas that are exposed to the front surface of the first body and to the rear surface of the second body as the first body and the second body are relatively moved; and a rear window unit detachably coupled to the rear surface of the second body, and configured to cover the flexible display exposed to the rear surface of the second body.