Provided is a failover method for a cluster system for realizing smooth failover of the guest OS's, even when there are many guest OS's, while reducing consumption of computer resources of a server. Smooth failover is realized by preventing competition during failover even when the number of guest OS's is increased. In a cluster configuration in which a slave/master cluster program is operated in a guest OS/host OS, the master cluster program (510) collects and transmits heartbeats of the slave cluster program, thereby realizing failure monitoring through the certain amount of heartbeats without depending on the number of guest OS's. Further, when the master cluster program monitors failures of the slave cluster program of its own computer to find a normal operation of the guest OS, the amount of communication through heartbeats is reduced by eliminating the necessity of communication to a standby system slave cluster program.