1. An image forming apparatus configured to print images of different document sizes on paper having a predetermined paper size, comprising: a computation processing part configured to compute a print position of the image with respect to the paper, based on the document size, the paper size and moving amount computation information that is input thereto; and a print processing part configured to arrange and print the image on the paper, based on the print position computed by the computation processing part, wherein the moving amount computation information includes a magnification of the image to be printed on the paper, a corner position that is used as a reference when computing the print position of the image to be printed on the paper, and an amount of margin at the corner position, and wherein the computation processing part sets the corner position on a back side of the paper when binding a longer side of the paper in a duplex printing to: a bottom left corner of the back side of the paper if a bottom right corner on a front side of the paper is set as the corner position on the front side; a top left corner of the back side of the paper if a top right corner on the frontside of the paper is set as the corner position on the front side; a bottom right corner of the back side of the paper if a bottom left corner on the front side of the paper is set as the corner position on the front side; and a top right corner of the back side of the paper if a top left corner on the front side of the paper is set as the corner position on the front side.