A conversion device receives service data including first connection destination data and the authentication information about an authenticated user, generates second connection destination data for designation of the first connection destination data, then associates the authentication information, the first and second connection destination data with one another, transmits them to a relay device, and transmits to a client the service data in which the first connection destination data is replaced with the second connection destination data; the client transmits the second connection destination data selected by the user to the relay device; the relay device transmits the authentication information to a server indicated by the first connection destination data using the authentication information and the first connection destination data corresponding to the second connection destination data, and transmits the address of the server to the client; and the client communicates with the server using the address and the authentication information.