In order to distribute multicast packets, using different multicast addresses and avoiding fault points, a backup server is installed close to a master and converts backup multicast packets and sends them as master multicast packets to the client. At this time, backup multicast packets are delayed and sent while the delay is determined from an allowable retransmission time, a one-direction delay to the client, a time required to detect linkage problems, and a time required to establish backup paths and so on.