PURPOSE: To stabilize the reverse charge characteristic of a photosensitive medium by using a magnet brush cleaning system, and by providing a member and a mechanism which make press-contact with and slide on the outer surface of the photosensitive medium, arbitrarily, independent from an image forming process.

CONSTITUTION: A photosensitive medium 1 is positive-charged by a PPC (precleaning charger) 3 just before a magnetic brush cleaning device 2. Further, a precleaning lamp (PCL) is provided on the back side of the photosensitive medium belt. Accordingly, a used part of the outer surface of the photosensitive medium is rubbed by a suitable member 5 so as to easily form an electron varier prior to use thereof, in order to avoid lowering the positive charge and deteriorating a cleaning function. Thereby, it is possible to readily and surely stabilized the reverse charge characteristic opposite to that of the main charge, and accordingly, it is possible to stabilize the cleaning function.