A plate-making system, in which a light-sensitive lithographic printing plate carrying an interleaf paper superimposed thereon and/or an original plate for making a dummy plate carrying an interleaf paper superimposed thereon are processed in a plate-setter provided with an automatic interleaf paper-removing mechanism and an automatic developing machine, is characterized in that it comprises a means for detecting the presence of any interleaf paper on the lithographic printing plate and/or the original plate or a means for discriminating the interleaf paper, the light-sensitive lithographic printing plate and the original plate. A plate-making method of the invention is characterized by irradiating the interleaf paper, the lithographic printing plate and the original plate for making a dummy plate with sensor light rays after they are passed through the automatic interleaf paper-removing mechanism and determining the intensity of the light to thus detect any interleaf paper possibly present on the plates or discriminate the interleaf paper, the light-sensitive lithographic printing plate and the original plate for making a dummy plate from one another. The plate-making method permits the reliable detection of the presence of interleaf papers on light-sensitive lithographic printing plates and/or original plates for dummy plates and the plate-making operations can thus stably be carried out using a plate-setter and an automatic developing machine.