PURPOSE: To simplify the changeover connection operation of a data terminal accommodated in the same terminal adaptor by adding a function by which the opposite terminal receiving a data terminal switching request signal to make changeover connection from a 1st data terminal to a 2nd data terminal to the system.

CONSTITUTION: A data terminal 3 accommodated in a terminal adaptor 1 makes data communication via an ISDN 7 with any of data terminals 4-6 accommodated by a terminal adaptor 2. Then a line switching request signal is set to a user information message in a signaling message on a D-channel from the terminal adaptor 1 and the set signal is sent to the terminal adaptor 2, and the terminal adaptor 2 receiving the lien switching request signal changes the connection from the data terminal 4 to, e.g. a data terminal 5. Thus, when it is required to make communication with other data terminal accommodated in the same terminal adaptor 2, the operation is simplified.