The reliability is improved at a low cost even in a virtualized server environment. The number of spare servers is reduced for improving the reliability and for saving a licensing fee for software on the spare servers. A server system comprises a plurality of physical servers on which a plurality of virtual servers run, a single standby server, a module for detecting an active virtual server, and a module for switching the correspondence of boot disks of virtualization modules for controlling virtual servers to the physical servers. When a physical server fails, the boot disk of the associated virtualization module is connected to a spare server to automatically activate on the spare server those virtual servers which have been active upon occurrence of the failure.