An exposing unit optically writes respective color-separated image data on a photosensitive drum to form a latent image by using both a forward direction scanning and a backward direction scanning. A writing control unit outputs input pixel data to the exposing unit a plurality of times for enlarging and printing an input image. The writing control unit includes an output unit that outputs a main-scanning effective gate signal of dot period lengths of common multiples of all available enlargement ratios; and a suppressing unit that suppresses an output of a pixel signal to an outside of a printing range.