Image-forming apparatus having external security paper detection unit and image-forming method thereof

An external security paper detection unit, an image-forming apparatus having the external security paper detection unit, and an image-forming method related to the apparatus. The external security paper detection unit includes a paper stack unit in which sheets of paper are stacked in order to feed the paper, a paper feed unit for feeding the paper stacked in the paper snick unit, a security paper detection unit for detecting whether moving paper is security paper or not, and a unit controller for normally supplying the moving security paper to a manual feed tray if the paper feed unit operates and a security paper detection signal is input from the security paper detection unit, whereby an image is formed only on the security paper, thus preventing the illegal leakage of documents.