PURPOSE: To reduce a job time by implementing loopback control of a text bypassed through between communication nodes at both ends.

CONSTITUTION: An extension command II is given to a node N 11 at the upperstream and an N13 at the lowerstream of a transmission line 55B between communication nodes N in which a new node 14 is extended and the N11 implements the control of loopback transmission to the transmission line 55B and the N13 implements the control of loopback transmission to a transmission line 55A. Thus, the text transmission to the N13 implemented from the N11 via the transmission line 55B is immediately lost and the new N14 is connected to the transmission line 55B between the N11 and N13 without interrupting the communication. After the end of the extension, the extension command II is released and the extension of the new N14 is finished as to the transmission line 55B. Thus, the extension or removal processing of a communication node is immediately implemented without interruption of the communication in operation and at the start of the loopback control to reduce the job time.