An image forming apparatus which forms an image by dot images. The invention includes a halftoning device for processing of halftone for inputted image data, a judging device for judging characteristics in the neighborhood of a target dot output from the halftoning means, gamma correcting means for correcting the gamma characteristic of image data of the target dot according to the judging by the judging device. Therefore, it is possible to prevent image degradation and to decrease a number of levels of halftone of the printed image.