A display device comprising: a transparent display unit configured to display a first screen in a foreground of the transparent display unit; a rear camera configured to capture a background object that is visible through behind the transparent display unit; and a controller configured to: set a transparency level of the first screen and apply the set transparency level to the first screen; recognize a region of the first screen on which the background object that is visible through the transparent display unit is displayed and calculate a value of distance between the background object and the transparent display unit; apply a transparency level to the recognized region based on the transparency level of the first screen and on the calculated value of distance, wherein the transparency level of the recognized region is higher than the transparency level of the first screen; and change at least the transparency level of the first screen or the transparency level of the recognized region in response to a display of a second screen on the transparent display unit; apply the transparency level of the recognized region to the second screen and increase the transparency level applied to the recognized region when the second screen is displayed behind the first screen; and apply the transparency level of the first screen to the second screen and increase the transparency level applied to each of the first screen and the recognized region when the second screen is displayed in front of the first screen. a transparent display unit configured to display a first screen in a foreground of the transparent display unit; a rear camera configured to capture a background object that is visible through behind the transparent display unit; and a controller configured to: set a transparency level of the first screen and apply the set transparency level to the first screen; recognize a region of the first screen on which the background object that is visible through the transparent display unit is displayed and calculate a value of distance between the background object and the transparent display unit; apply a transparency level to the recognized region based on the transparency level of the first screen and on the calculated value of distance, wherein the transparency level of the recognized region is higher than the transparency level of the first screen; and change at least the transparency level of the first screen or the transparency level of the recognized region in response to a display of a second screen on the transparent display unit; apply the transparency level of the recognized region to the second screen and increase the transparency level applied to the recognized region when the second screen is displayed behind the first screen; and apply the transparency level of the first screen to the second screen and increase the transparency level applied to each of the first screen and the recognized region when the second screen is displayed in front of the first screen. set a transparency level of the first screen and apply the set transparency level to the first screen; recognize a region of the first screen on which the background object that is visible through the transparent display unit is displayed and calculate a value of distance between the background object and the transparent display unit; apply a transparency level to the recognized region based on the transparency level of the first screen and on the calculated value of distance, wherein the transparency level of the recognized region is higher than the transparency level of the first screen; and change at least the transparency level of the first screen or the transparency level of the recognized region in response to a display of a second screen on the transparent display unit; apply the transparency level of the recognized region to the second screen and increase the transparency level applied to the recognized region when the second screen is displayed behind the first screen; and apply the transparency level of the first screen to the second screen and increase the transparency level applied to each of the first screen and the recognized region when the second screen is displayed in front of the first screen.