PURPOSE: To control each relay accommodating processor to a proper operating rate by informing a congesting state of a processor accommodating a relay line of each node to a relay line selector by means of a congestion control signal.

CONSTITUTION: If a relay line accommodating processor P2 of a node N2 reaches a congesting state, the congestion controller informs a congestion control signal S to all nodes. Then when a call request from a terminal device T1 to a terminal device T2 is received by the node N1, the relay line selector uses a control signal S from the node N2 to disuse the relay line L2 and the relay line L1 or L3 is selected. In such a case, since the relay line selector knows that the processor P1 is not yet congested among the relay line accommodaing processors P1, P2, the selector grasps the relay line L1 as usable one and the flexibility on network operation is assured.