When a multiple-color image is printed by electrophotography, unwanted spaces are produced between different colors. These spaces are attributed to non-development of toner caused by the involvement of an electric field at the boundary of an electrostatic latent image formed on a photosensitive drum. Accordingly, when a data conversion is made from two-bit data of the bitmap to eight-bit data that are for modulating a light signal, a pixel of interest and the neighboring 9 × 9 pixels are stored and reference is made to the state thereof. If the pixel of interest is a white pixel and the neighboring pixels are all non-white pixels, then the pixel of interest is not left at a value corresponding to white but is converted to a value corresponding to such a potential that will not develop toner. The photosensitive drum is irradiated with a laser based upon the data resulting from the conversion. Thus, a sudden conversion of potential from a potential corresponding to white to a potential corresponding to the maximum density is eliminated at the outline of the image. This makes it possible to prevent the involvement of an electric field at the edge of the image and, hence, to prevent the occurrence of unwanted spaces between colors.