RELAY DEVICE, NETWORK SYSTEM, AND NETWORK FAILURE DETECTION METHOD

A relay device comprises: a packet type determining unit that determines: whether a received packet is a monitoring packet, sent by a first communication device that monitors a network connection state, set the first communication device as the sending source, and set a second communication device that is monitored as the sending destination; and whether the received packet is a monitoring response packet sent as the response to the monitoring packet with sending source address and destination address of monitoring packet interchanged; a transfer processing unit that receives the monitoring packet and transfers it to a destination; a monitoring unit that monitors receipt of the monitoring response packet within a specified period; and a failure notification packet sending unit that generates a failure notification packet and sends it to a specified destination, when the monitoring response packet is not received within the specified period.