In a server-client type system, when a large volume of data to be transmitted whose volume is not less than a predetermined value is generated, a server notifies to a terminal device as a transmission destination to the effect that the large volume of data is to be transmitted by the D-channel packet exchange, and after receiving a data transmission timing signal from the terminal device, starts transmission of the large volume of data using a B-channel to cause the terminal device to download the large volume of data, and the terminal device monitors a state of a free B-channel line in all ISDN communication devices on the client side connected to the same digital service unit upon receiving the notification of transmission of the large volume of data from the server and notifies the server of the data transmission timing signal by the D-channel packet exchange at timing not preventing use of a B-channel line.