An objective of the present invention is to secure the continuity of the communication between a client device and a standby server device, when a failure occurs and switch to the standby device is performed. A communication processing unit of the client device establishes connection with a plurality of server devices simultaneously by transmitting a UDP packet to the plurality of server devices by multicast. After establishing the connection, data of the application is transmitted by multicast to the plurality of the server devices. In the even of a failure, a standby server device receives the data packet multicast from the client device to continue the inter-application communication.