An image forming apparatus for depicting a gradation image through halftones includes a threshold value generating unit for generating a threshold value matrix, and a comparison unit for converting the gradation image. The threshold value generating unit determines an order of gradations of pixels in halftones. The threshold value generating unit determines an order of standard dots. Further, the threshold value generating unit is configured to determine a priority order of gradations between the halftones. The threshold value generating unit further determines an order of the threshold values of the gradations of all of the pixels according to the order of the gradations of the pixels contained in the halftones and the priority order of the gradations between the halftones, so that the threshold value generating unit generates the threshold value matrix.