An image forming apparatus and method which can take a countermeasure against a tail or the like in consideration of differences of various conditions such as a kind of transfer paper and the like is provided. The apparatus has an image processing portion for extracting an image pattern whose image information has a concentration of 100% and executing a thin-out process for thinning out a predetermined area in the extracted image pattern at a predetermined ratio. The image processing portion or step changes the thin-out process according to the transfer paper kind.