PURPOSE: To prevent the ground of a transferred material from being stained and to reduce the size and cost of the device by detecting light and shade corresponding to an electrostatic charging potential outside an area of a photosensitive body after specific development and correcting a bias electric field according to variation of the detection signal.

CONSTITUTION: The development is performed by using a developing bias with a potential VB which is nearly equal to or higher than the electrostatic charging potential VD at a part other than the electrostatic latent image formation area of the photosensitive body 2. Then the light and shade corresponding to the electrostatic charging potential VD is detected by a light emitting element 12 and a photodetector 13 to obtain an electric detecting signal and the electrostatic charging potential VD is corrected according to the variation of the detection signal. As for an adjusting method by this constitution, the detection signal varies corresponding to the potential VD which drops gradually after the frequency of image formation reaches a constant value, but the bias electric potential VB is made constant and the potential VD is corrected to correct a bias electric field, so when a toner image is transferred to a material 6 for transfer, the ground part is never stained. Therefore, no potential sensor is required and the device is reducible in size and cost.