In a communication system, transmission/reception of data can be performed between end hosts connected to a relay switch in a subnet even in an improper connection at a L2 level in the relay switch in a communication. Communication failure due to a packet loop generated by the improper connection of the L2 level is recovered by wherein the packet loop is stopped by continuously transmitting long-size MAC packets longer than a packet generating the packet loop into the subnet, which have destinations as a broadcast MAC address or multicast MAC address.