An image forming apparatus provided with a photoreceptor with an internal gear includes the internal gear in a photoreceptor drum, a pinion gear in engagement with the internal gear, and a cleaning unit and a developer unit which apply contact pressure onto the photoreceptor drum. The pinion gear is provided in such an area that a resulting force of the contact pressure respectively applied from the process elements does not cause a distortion of the photoreceptor drum which exceeds an axial backlash determined in initialization according to a distance between axes of the pinion gear and the internal gear. The described arrangement prevents interference between non-driving surfaces of the internal gear and the pinion gear. As a result, a smooth rotary motion of the photoreceptor drum can be ensured, thereby preventing an image defect caused by an unstable rotary motion of the photoreceptor drum.