1. An image forming apparatus, comprising: a contact glass on which a sheet of original paper is placed; an image acquiring unit for reading the original paper and acquiring an original image; a printing unit for printing in color an original image acquired by the image acquiring unit on a sheet of recording paper, using a plurality of colored toners; a storing unit for storing a specific pattern which expresses, using a dot pattern, apparatus identification information for identifying an apparatus which prints the original image on a sheet of recording paper; and an extracting unit for extracting an actual image area except a blank area in the original image, and based on the extracted actual image area, extracting a specific area corresponding to an area for printing the specific pattern, wherein the printing unit prints the specific pattern within the specific area, using a predetermined colored toner for printing the specific pattern, and wherein if a first original paper and a second original paper are placed at a predetermined interval on the contact glass: the image acquiring unit acquires a first original image and a second original image which correspond to the first original paper and the second original paper; the extracting unit extracts an actual image area except a blank area in each of the first original image and the second original image, and based on the extracted first actual-image area and second actual-image area, extracts a first specific area and a second specific area which correspond to an area for printing the specific pattern; and the printing unit prints the specific pattern within each of the first specific area and the second specific area, using a predetermined colored toner for printing the specific pattern.