In a cluster system including a plurality of operating systems operating on one
computer, computer resources can be updated for and reallocated to each operating
system. When the operating systems are used as active or standby operating systems,
a multiple operating system management controller monitors the state of each operating
system. At a failure of an active operating system, the controller allocates a
larger part of computer resources to another operating system in a normal state
and assigns the operating system as a new active operating system. Regardless of
the failure, the computer system can be operated without changing processing capability
thereof. The controller can monitor load of each operating system to allocate computer
resources to the operating system according to the load.