A center site is disposed to intervene in a business transaction achieved through a network. The center site includes an open business information database in which open business information received from member sites connected to the network is accumulated to be opened to the sites and a notarization database to keep therein contents of contracts of transactions between the sites. The center site receives a transaction request from a transaction partner site in accordance with the open business information and notifies the request to an information supply site associated therewith. The center site intervenes in a transaction resultantly accomplished between the information supply site and the transaction partner site and conducts a notarization process for the contents of contract for the transaction to accumulate the contract in a notarization database.