PURPOSE: To allocate lots of terminal devices without increasing the number of transmission slots of a frame and to transmit a data efficiently by using in common each data transmission slot for data transmission among plural terminal devices in exchange connection.

CONSTITUTION: A loop transmission line 1 is provided with plural nodes N, and lots of data terminal devices T and voice terminal devices TEL are accommodated to each node N. In applying exchange connection for the data terminal devices T, a preparation of call is requested from the terminal device T to the node N accommodating it. A sender node NS transmits an address of the terminal device T and a transmission request signal to a header of the transmission line 1 and awaits a response from the reception side. A reception side node ND stores a sender side node number, an address of the sender side terminal device T and an address of the terminal device T accommodated by itself and transmits a ready signal representing that the terminal device is receptionable when the terminal T of the reception side is idle. When the response to the transmission request given from the node NS is received, the sender node NS informs the end of connection to the terminal device T.