The invention provides an apparatus for forming a latent image on a photoreceptor. In the apparatus, there are provided an image reader including a scanning device for performing a reciprocal movement along an original document, a reading device for reading an image on the original document and for outputting an image signal in the outward movement of the scanning device, and a memory for storing the image signal; and an image writing device for writing a latent image on the photoreceptor. The image writing device has a first writing mode in which a latent image corresponding to the image signal outputted from the reading device is written in synchronization with the outward movement of the scanning device, and a second writing mode in which a latent image corresponding to the image signal outputted from the memory is written. The scanning device is adapted to perform the backward movement in the second writing mode.