A frequency modulation device for use in an image forming apparatus. The image formation apparatus includes an image carrier and a laser device for scanning the image carrier along a plurality of scan lines. Each scan line is divided into segments having segment boundaries in which the same segment boundary in adjacent scan lines are offset. The frequency modulation device generates frequency data for use in modulating the input image data, which is utilized by the laser device to scan the image carrier, which permits output of an electrophotograph of high image quality by suppressing segment boundaries caused by moiré fringes or color shifting to below a level at which such boundaries are not visually detectable.