A display device including an interface unit configured to be connected to a mobile terminal having a first display unit displaying a mobile screen image; a second display unit for displaying a monitor window having two orientation directions; and a controller configured to display the monitor window in a first orientation direction for displaying a monitor screen image corresponding to the mobile screen image on the second display unit; receive a user command relating to a modification of the mobile screen image; transmit a control signal relating to the user command to the mobile terminal; and display a modified monitor screen image in response to the user command on a second orientation direction of the monitor window.