The image forming apparatus includes: an image data input section for reading a document as image data; a target value memory section for storing a target value of an input/output characteristic value when a reference document is read; a comparator for comparing the image data of the reference document and the target value so as to create a new input/output characteristic value; a fixed value memory section for storing a difference in input/output characteristic values as a fixed value between an original document and a copy document; and an adder for adding the fixed value to the image data of the reference document so as to output a resultant value to the comparator. By the provision of the fixed value memory section and the adder, the image forming apparatus can set a plurality of input/output characteristic values by reading the reference document once, thus reducing time for the procedure of setting the respective input/output characteristic values.