A display device, comprising: 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; and receive a user command relating to a modification of the first orientation direction of the monitor window to a second orientation direction; characterized in that the controller is further configured to: transmit a control signal relating to the user command to the mobile terminal, wherein the mobile terminal controls an orientation direction of the mobile screen image to be changed to the second orientation direction according to the control signal even though a housing of the mobile terminal keeps being oriented in the first orientation direction, and the mobile terminal controls the mobile screen image to be modified according to the second orientation direction; receive a modified mobile screen image corresponding to the user command from the mobile terminal; and display a modified monitor screen image corresponding to the modified mobile screen image on the second orientation direction of the monitor window. 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; and receive a user command relating to a modification of the first orientation direction of the monitor window to a second orientation direction; characterized in that the controller is further configured to: transmit a control signal relating to the user command to the mobile terminal, wherein the mobile terminal controls an orientation direction of the mobile screen image to be changed to the second orientation direction according to the control signal even though a housing of the mobile terminal keeps being oriented in the first orientation direction, and the mobile terminal controls the mobile screen image to be modified according to the second orientation direction; receive a modified mobile screen image corresponding to the user command from the mobile terminal; and display a modified monitor screen image corresponding to the modified mobile screen image on the second orientation direction of the monitor window. transmit a control signal relating to the user command to the mobile terminal, wherein the mobile terminal controls an orientation direction of the mobile screen image to be changed to the second orientation direction according to the control signal even though a housing of the mobile terminal keeps being oriented in the first orientation direction, and the mobile terminal controls the mobile screen image to be modified according to the second orientation direction; receive a modified mobile screen image corresponding to the user command from the mobile terminal; and display a modified monitor screen image corresponding to the modified mobile screen image on the second orientation direction of the monitor window.