Abstract: A network system includes a plurality of printers and a digital copying machine interconnected through a data transmission line, detects a free space in an image memory of each printer, adds a user identification code to image data of a document read in an image reading section of the digital copying machine, and then sends the image data to a printer with the image memory having a sufficient free space so as to store the image data. With this structure, even if the digital copying machine does not have an image memory, it is possible to store the image data subjected to an image formation within the network and omit an image reading process when performing an image formation according to the same image data later. Moreover, with this structure, various processing can be executed on the image data by effectively using the functions of the respective apparatuses.