The guest cluster programs
2007 and
2017, or
2037 and
2047 monitor operation situations of application programs
2006 and
2016, or
2036 and
2046, and give a notice of a result of the monitoring to the host cluster program
2003 or
2033. Upon receiving a system switching instruction from the host cluster program
2003, the guest cluster programs
2007 and
2017 execute system switching of a master/slave mode for their respective corresponding virtual machines
2005 and
2015. When the virtual machine is in the master mode, the application conducts processing on input data and outputs a result of the processing. On the other hand, when the virtual machine is in the slave mode, the application conducts processing on input data, but it doesn't output a processing result. Upon receiving a system switching instruction from the host cluster program
2033, the guest cluster programs
2037 and
2047 also execute system switching of a master/slave mode for their respective corresponding virtual machines
2035 and
2045 in the same way. On the other hand, the host cluster program
2003 or
2013 monitors operation situations of virtual machines operating on the same physical computer (the virtual machines
2005 and
2015 or the virtual machines
2035 and
2045). For example, upon detecting that the virtual machine A
2005 in the master mode is suspended, the host cluster program
2013 switches the virtual machine A
2035 from the slave mode to the master mode.