Synchronizing client data and server data

To synchronize client data and server data, while reducing the number of transactions, a server has a sending-confirmed information storage section which stores, as a sending-confirmed list, content last confirmed in a client; an update data set storage section which stores the updated data set to be held by the client; an update information sending section which sends the difference between a data set identified from the sending-confirmed list and an updated data set; a sent information updating section which stores in a sent information storage section content of the updated data set as a sent list; an update request receiving section which receives an update request from the client; and a sending-confirmed information updating section which replaces the sending-confirmed list with the sent list if the content of the data set held by the client is different from the content of the data set identified from the sending-confirmed list.