A representative gradation value is set collectively for each pixel group of several pixels each, and then the representative gradation value undergoes multilevel halftoning. From the multilevel halftoning result value obtained in this way, the presence or absence of dot formation for each pixel is set and the image is output. Also, based on the resolution ratio of the image data input resolution and output resolution, if the pixel group is divided, it is possible to divide the pixel group into suitable sizes for processing, so it is possible to improve the output image quality. Furthermore, even when the pixel group is divided, it is possible to share data for processing, so it is possible to save memory. In addition, it is possible to rapidly output high quality images with a simple process.