An image density control device is for use in an image forming apparatus having an exposure lamp for emitting light to illuminate a document, a photoreceptor for receiving light reflected from the document to produce an electrostatic latent image, and a charger for charging the photoreceptor. The image density control device includes a detector for detecting the density of the document, a light emission amount controller responsive to the detector for holding an amount of light emitted from the exposure lamp at a predetermined fixed value when the document density lies in a predetermined range, and surface potential controller responsive to the detector means for controlling the charger to change the surface potential of the photoreceptor in accordance with a change in the document density when the document density lies in the predetermined range. The predetermined range may be a range defined by a minimum document density and a reference document density, or the one defined by another reference document density and a maximum document density. In the former case, the surface potential is increased in accordance with a decrease in the document density when the document density lies in the predetermined range. In the latter case, the surface potential is decreased in accordance with an increase in the document density when the document density lies in the predetermined range.