PURPOSE: To attain the transmission of an economical broad band signal by lowering the connection time of the broad band signal in comparison with the communication time.

CONSTITUTION: When subscribers 2, 6 perform 5min communication by using a narrow band signal and if the connection of the broad band signal such as a picture is required for 30sec on its way, a broad band connection request signal is sent from the subscriber 2, connection control circuit 24, 25 control connection equipments 22, 23 of a connection controller to connect the broad band transmission line for a required time. The time required for connection/ disconnection is a very short time, because the subscribers 2, 6 are connected already by the narrow band circuit and the procedure (dialing, calling or reply) to connect both of them is omitted.