A downstream server stores product information on a radio-frequency (RF) tag outputted from a reader/writer into a database. A processing unit executes various processing using product information stored in the database. If product information necessary for the processing unit to execute various processing is not stored in the database, a transmission request for the product information is outputted to an upstream server. The upstream server transmits the product information stored in a memory unit to the downstream server. The upstream server stores the history of received transmission requests for product information in a history database. The upstream server determines priority rankings for product information, based on the history stored in the history database. The upstream server outputs a predetermined number of pieces of product information having higher priority rankings determined to a reader/writer as information to be written on a RF tag.