When an user inserts already used paper into a shredder of PC, a scanner disposed with the shredder reads a background pattern put on the paper. The shredder shreds and discards the inserted paper. The PC transmits the background pattern put on the discard paper to a background management unit and further notifies a background management unit that the paper is discarded. The background management unit places the background pattern sent from the PC in an unused state and manages the background pattern so that the background pattern can be reused.