PURPOSE: To always stably obtain an adequate copied image without having the influence of a toner staining on a sensible image density detecting means by changing the set value of a lamp voltage with respect to an original density in accordance with the value of the ratio between a measured value when there are no sensible images and the measured value of the sensible image density when there is the sensible image and the value of the lamp voltage of an exposing lamp.

CONSTITUTION: A control means D changes the set value of the adequate lamp voltage with respect to the original density in accordance with the sensible image density of medium contrast and the value of the lamp voltage when said density is measured from the original density and sensible image density applied from respective detecting means A, B. The exposure at the time of reproducing the original image is thus determined by correcting the adequate lamp voltage unequivocally determined according to the original density according to the change in the conditions to apply a fluctuation thereto. On the other hand, the control means obtains the measured value when there are no sensible images on a photosensitive body and the sensible image density when there is the sensible image thereon by the sensible image density detecting means and makes correction with the ratio therebetween as the information on the sensible image density and, therefore, the error in the density detection of the sensible image density detecting means by the toner staining is eliminated from said information in the sensible image density.