An image forming apparatus includes a photoreceptor having on a surface thereof a photosensitive layer, in which an electrostatic latent image is formed, a contact charging roll applied with a bias containing a direct current component having superimposed thereon an alternating current component, the contact charging roll charging the photoreceptor to a predetermined potential, a film thickness detecting unit detecting a film thickness of the photosensitive layer of the photoreceptor, an environment detecting unit detecting environment, and a contact area changing unit changing a contact area between the contact charging roll and the photoreceptor based on detected results of the film thickness detecting unit and the environment detecting unit.