A cluster system including: a transmission side server cluster consisting of a plurality of computers; a receiving side server cluster consisting of a plurality of computers; and a network that interconnects both the transmission side server cluster and the receiving side server cluster, wherein an active-transmission computer which is included in the transmission side server cluster selects a standby-transmission computer from the computers in the transmission side server cluster, based on load information, and transmits a backup-copy of a message to the standby-transmission computer when the active-transmission computer transmits the message to a computer in the receiving side server, the stand-by transmission computer for back-up handling of the message in the event of occurrence of a fault in the active-transmission computer.