An image forming apparatus for use in a network system including a network and a plurality of print-data-outputting apparatuses which sends via the network print-data including identification information identifying the print-data-outputting apparatus to the image forming apparatus for outputting a hardcopy of the print-data. The image forming apparatus receives print-data from each of a plurality of print-data-outputting apparatuses via the network, recognizes characteristics of the received print data, and judges whether reproduction of the received print-data is legally forbidden. If the received print data is forbidden, the image forming apparatus alters the received print-data so as to cause the fidelity of a reproduction of the received print-data to be lost or deteriorated and adds to the altered print-data identification information identifying the specific print-data-outputting apparatus sending the print-data. An image of the altered print-data and an image of the added identification information is printed on a sheet of paper and output as a hardcopy. A computer readable medium stores instructions for controlling an image forming apparatus used in a network system including a network and at least one print-data-outputting apparatus which sends via the network print-data including identification information identifying the print-data-outputting apparatus sending print-data to the image forming apparatus for outputting a hardcopy of the print-data.