A raster device such as a digital printer or the like which sequentially compares input image data with each element in a threshold matrix to convert the input image data into binary or multi-level pixel data. For providing both a higher resolution and a larger number of halftone levels for an output image at a low cost, an input image is decomposed into four color planes as CMYK data, and data on each color plane is compared with a dither matrix in a binarization circuit to be converted to binary data. The dither matrix generates a dither pattern composed of eight clusters having substantially the same shape as a kernel cluster capable of expressing 15 or more halftone levels. In this way, a smooth gradation having 120 or more halftone levels is realized by repetitively applying a small pattern having approximately 15 halftone levels.