A backup system is configured by interconnecting a backup management device and disk array device via a SAN. The charge roller includes a surface having protrusions and depressions and to electrically charge the image carrier. The particle stream is preferably formed coaxially around the laser beam, the particles being accelerated by means such as an inert gas or electrostatic charge which does not work the material. The minimum value of the three intensity words for each pixel is determined and used as an initial black value.