PURPOSE: To improve the transfer quality by keeping the packets in the reception confirmation signal reception waiting state from each inter-station signal device into a queue and resending packets in the queue to ah auxiliary inter- station signal device at the time of trouble of the inter-station signal device.

CONSTITUTION: Packets for which the reception confirmation signal from inter- station signal devices 3 and 4 are not received out of packets transmitted to signal devices 3 and 4 are kept in queues 1; and when the reception confirmation signal is received from the inter-station signal device 3 or 4, the packet corresponding to this signal is released from the queue 1 corresponding to this inter- station signal device 3 or 4. If the inter station signal device 3 is faulty, packets in the queue 1 corresponding to this inter-station signal device 3 are kept in an auxiliary queue 2 and an auxiliary inter station signal device 6 is initialized, and packets are retransmitted to the auxiliary inter-station signal device 5 after the link of an auxiliary inter-station line 6 is set.