PURPOSE: To improve the efficiency of a communication network by monitoring an input port by means of a signal discriminating means at a closed down port and, when a certain communication signal is received, restoring the down port to the normal state by means of a down port controlling means.

CONSTITUTION: A signal pattern detecting section 11 always monitors input signals and, when a certain signal is inputted, sends a control signal to a DWN controlling section 13 to restore a down port to the normal state. A timer 14 monitors the status of an input and output ports during broadcasting and, when a no-signal status continues for more than fixed time, the timer 14 becomes time-out. As a result, a path is opened. The time-out information is outputted to a sequencer 9 and the sequencer 9 sets the drivers 18 of all ports to enable states so as to open the path. Therefore, an acknowledge signal ACK can be accepted at a certain input port at the moment an identification signal AF is inputted.