1. An image forming method for variable printing, comprising: receiving, in an image forming apparatus, a print job for variable printing indicating two or more kinds of attributes; associating, in the image forming apparatus, information regarding an attribute from the two or more kinds of attributes with variable print contents on a top surface of a sheet of paper such that sheets with like variable print contents are associated with the same attribute from the two or more kinds of attributes; printing a character forming image of the attribute at a first position in a top-end part of each sheet of a plurality of sheets of paper, which are to be sorted by the attribute, by slicing a character that represents the attribute, based on a numerical position of the sheet of paper within a paper group sorted by the same attribute, wherein said character forming image for each sheet is obtained by sequentially slicing the character in a predetermined direction at a predetermined interval such that the plurality of sheets, when printed and stacked, displays, on a side-face of the printed stack of plurality of sheets, the character representing the attribute, for each of the two or more kinds of attributes.