1. An image forming method, comprising: printing a character forming image in an end part of each sheet of a plurality of sheets of paper which are to be sorted by an attribute, in an order of slicing a character which expresses the attribute of each sheet, based on what number sheet of paper is each sheet among a paper group which is sorted by the same attribute, the character forming image being obtained by sequentially slicing the character in a previously determined direction at a previously determined interval.