A speed change detection apparatus includes an image carrier for carrying a latent image, and a latent image pattern forming device for forming a latent pattern image on the image carrier. The pattern image includes periodically formed line or dot images. An alternating current conversion type surface potential sensor is provided to detect a potential of the surface of the photoconductive member. The alternating current conversion type surface potential sensor further detects a change in speed of the image carrier by detecting a moiré appearing on the latent image pattern.