the present invention is directed to the provision of an image forming apparatus capable of effectively preventing the formation of images related to illegal conducts by making provisions so that , even in situations where images can be formed based on image data outputted from any arbitrary image output apparatus , the image output apparatus which outputted the image data can be identified without fail . when image data is inputted from a portable image output apparatus , first it is checked whether identification information has been inputted together with the image data , then an image of an identification pattern is created , and image formation is performed based on image data obtained by attaching the image data of the identification pattern to the input image data . on the other hand , when image data is inputted from a fixed image output apparatus , first it is checked whether identification pattern data is included in a designated position in the image data , and then image formation is performed based on the input image data . the image thus formed by the digital copying machine contains the image of the identification pattern by which the image supply device that outputted the source image data is identified .