A scan ID, which is operation identification information having been used for selection of an image reading condition or the like, is used for selection of a communication destination from among a plurality of host PCs having network-connection to a scanner relay apparatus. Information such as an image reading condition associated with the scan ID is sent to a host PC defined as a communication destination, and a scan command sent back from the host PC, whose protocol is converted, is sent to a scanner apparatus. Furthermore, image data sent back from the scanner apparatus is sent to the host PC.