PURPOSE: To always keep a steady reproducibility of an image with a half tone and obtain an image free from fog, by detecting the latent image potential of an image of half tone and applying the development bias voltage almost equal to the detected latent image potential to the development electrode.

CONSTITUTION: An electrophotographic copier consists of a photosensitive drum 1 that carries resin photoreceptor of CdS.nCdCO3 on its surface, an original plating plate 2, a reference chart 3 provided at the non-image part of the plate 2, an original illuminating light source 4, a photographing lens 5, an electric charger 6, a magnetic brush developing device 8, a latent image potential detecting means 9, a development bias power source 10, a transfer charger 11, a cleaning blade 12 and a remaining charge eraser lamp 13. The chart 3 is gray of a low density, and the chart potential is detected by the detector 9. Then the bias voltage corresponding to the density of the chart 3 is applied to improve the reproducibility of a copied image accompanying to the repetition of the copying action.