A server system including a plurality of servers achieves redundancy on a per process basis. A first server and a second server execute a plurality of processes in parallel. Processes (B1-1, B2-1, and Br-1) in the first server are active processes, and processes (B1-2, B2-2, and Br-2) in the second server are standby processes. The active process periodically transmits an operation notification notifying normal operation, to the standby process. When no longer receiving an operation notification from the active process, the standby process switches an operation mode of the standby process from a standby mode to an active mode and starts provision of service of the process. The process switching is performed on a per process basis instead of on a per server basis.