PURPOSE: To prevent the short circuit between a photosensitive body and a developing roller by a process not using a corona discharger and to form an image at a high speed by covering the surface of a conductive sleeve where a bias voltage is impressed with an insulating layer, or a resistance layer, as to a developing roll provided with the conductive sleeve and a magnet installed inside the sleeve.

CONSTITUTION: A photosensitive belt 1 is formed by laminating a transparent conductive layer 13 and an organic photoconductive layer 14 on a transparent supporting body 12 such as polyethylene terephthalate, etc. A developing device 4 arranged opposite to the photoconductive layer 14 and for storing toner inside is provided with the developing roll 5 constituted of a magnet roll 17, the conductive sleeve 16 and the insulating cover 15, conductive toner whose resistivity is ≤109Ωcm is attracted and held by the magnetic force of the magnet roll 17 so as to form a toner layer 6 on the insulating cover 15. A magnetic brush is formed by the toner in a toner contact area 18 where the toner layer 6 comes in contact with the photoconductive layer 14. By selecting the thickness of the insulating layer, the resistivity and inductivity in such a way, the bias voltage is distributed on the photoconductive layer, and then, charges are injected from the toner.