1. An image forming apparatus which forms an image by dot images comprising: halftoning means for processing of halftone for inputted image data; judging means for judging characteristics in the neighborhood of a target dot output from said halftoning means; gamma correcting means for correcting gamma characteristic of image data of the target dot according to judging by said judging means wherein said gamma correcting means includes storing means for storing at least two gamma correcting data, selecting means for selecting gamma correcting data among gamma correcting data stored in said storing means according to judging by said judging means and output means for outputting a writing level signal which is set according to the gamma correcting data selected by said selecting means, said apparatus further comprising pattern forming means for forming a dot pattern for gamma correction, a pattern reading means for reading density of said dot pattern formed by said pattern forming means and generating means for generating gamma correcting data according to said density of dot pattern read by said pattern reading means.