1. A speed change detection apparatus, comprising: an image carrier configured to carry a latent image; a latent image pattern forming device configured to form a latent pattern image on the image carrier, said pattern image including periodically formed line or dot images; and an alternating current conversion type surface potential sensor configured to detect a potential of the surface of the photoconductive member; wherein said 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.