1. A display tag comprising: a sensing unit which captures an image displayed on a screen of an external tag writer, if the sensing unit is disposed opposite to the screen, wherein the sensing unit comprises a plurality of sensors which constitute a sensing area, and receives light rays of the image displayed on the screen of the external tag writer and outputs a signal corresponding to intensities of the light rays; and a display unit which receives the signal from the sensing unit and displays the captured image, wherein the display unit comprises a plurality of pixels corresponding one-to-one to the plurality of sensors.