A sender device 1 stores, in header information about image information, information indicating that attribute information and image data rearranged in sequence, and produces image information by means of storing attribute information and image data in a predetermined sequence. The thus-produced image information is sent to a receiver device 2 by way of a network line 3. When receipt of image information is initiated, the receiver device 2 refers to the header information, thereby determining presence/absence, in the header information, of information indicating that attribute information and image data are stored in a predetermined sequence. When it has been determined that attribute information and image data are arranged in a predetermined sequence, image data included in the received image information are immediately decompressed, and the thus-decompressed image data are output in the form of a record or display.