In a general aspect, a foldable display device may include processor, a memory, a first housing having a first end portion and a second end portion, a first length extending between the first end portion and the second end portion of the first housing, a second housing having a first end portion and a second end portion, a second length extending between the first end portion and the second end portion of the second housing, the second length being different than the first length, a flexible display disposed on the first housing and the second housing, and a hinge assembly disposed between the first housing and the second housing to rotate the first housing and the second toward each other. The hinge assembly may include a slider and plurality of gears. The slider and the plurality of gears operate to transfer a rotational movement into a translational movement of at least one of the first housing or the second housing.