A display device, a mobile terminal, and a method for displaying a second screen image are discussed. The display device according to one embodiment includes an interface unit configured to be connected to a mobile terminal having a first display unit displaying a first screen image in a first display direction; a second display unit; and a controller configured to receive information related to the first screen image, and display a monitor window for displaying a second screen image corresponding to the first screen image on the second display unit. The second screen image is displayed in the first display direction. The controller is further configured to receive a user command to intend the monitor window to be oriented in a second display direction and control the monitor window to be oriented in the second display direction according to the user command, and transmit a control signal.