PURPOSE: To eliminate ground stains and to improve poor reproducibility, by detecting the surface potential of the latent image of a pattern having medium contrast and controlling and correcting the voltage to be applied on an exposing lamp.

CONSTITUTION: The output of the latent image potential of a pattern from an electrometer 11 is inputted to a controller 18, by which said output is compared with a standard potential and the voltage to be applied on an exposing lamp 3 is controlled by the difference between the same. A sensor pattern 2, an eraser 12, and a P-sensor 14 are used for controlling the density of a picture image based on a P-sensor method, and the pattern 2 is exposed on a photoreceptor 9 by an image exposing device to form a latent image which is developed with a developing device 13 and the pattern image is formed. The reflectivity thereof is detected with a P-sensor 14 and the replenishing of a toner to the device 13 is controlled, whereby the density of the image is controlled.