A display device comprising: a first display part; a second display part; and a third display part disposed between the first display part and the second display part, wherein the first display part is transparent, and the second display part is opaque, wherein the third display part comprises a transparent part and an opaque part, which are separated from each other characterized in that an area ratio of the transparent part with the opaque part in the third display part is gradually changed, the display device is configured to be folded at the third display part, wherein the third display part is connected to the first display part and to the second display part and the third display part is defined by adjacent portions of the first display part and the second display part, and by a boundary therebetween, wherein the boundary does not define a straight line along an entire length of the third display part, wherein the area ratio of the transparent part with the opaque part in the third display part is gradually increased or gradually decreased toward the second display part from the first display part, wherein the first display part further includes a polarizing layer disposed between a display panel and a window layer, wherein the polarizing layer decreases a reflectivity of light incident from an outside, and the polarizing layer is configured to provide a privacy function of the display device with respect to a user, wherein a viewing angle of the first display part is decreased by the polarizing layer or the visibility of the first display part is lowered by the polarizing layer, and when it is sensed that the display device is in the folded state corresponding to when it is sensed that the first display part is exposed to the top of the second display part, the display device is configured to control the first and the second display parts (11, 12), wherein the display device is configured to control brightness of the first display part to be dark, to control color temperature of the first display part to be a yellow tone, to display a resolution of an image displayed on the first display part to be lowered, or to display a size of the image to be reduced. a first display part; a second display part; and a third display part disposed between the first display part and the second display part, wherein the first display part is transparent, and the second display part is opaque, wherein the third display part comprises a transparent part and an opaque part, which are separated from each other characterized in that an area ratio of the transparent part with the opaque part in the third display part is gradually changed, the display device is configured to be folded at the third display part, wherein the third display part is connected to the first display part and to the second display part and the third display part is defined by adjacent portions of the first display part and the second display part, and by a boundary therebetween, wherein the boundary does not define a straight line along an entire length of the third display part, wherein the area ratio of the transparent part with the opaque part in the third display part is gradually increased or gradually decreased toward the second display part from the first display part, wherein the first display part further includes a polarizing layer disposed between a display panel and a window layer, wherein the polarizing layer decreases a reflectivity of light incident from an outside, and the polarizing layer is configured to provide a privacy function of the display device with respect to a user, wherein a viewing angle of the first display part is decreased by the polarizing layer or the visibility of the first display part is lowered by the polarizing layer, and when it is sensed that the display device is in the folded state corresponding to when it is sensed that the first display part is exposed to the top of the second display part, the display device is configured to control the first and the second display parts (11, 12), wherein the display device is configured to control brightness of the first display part to be dark, to control color temperature of the first display part to be a yellow tone, to display a resolution of an image displayed on the first display part to be lowered, or to display a size of the image to be reduced.