1. A computer system capable of connecting a plurality of physical computers to an I/O device through an I/O switch and sharing the I/O device; wherein the plurality of physical computers each include a plurality of LPARs built on the physical computers, and a plurality of virtual machine monitors for managing the plurality of LPARs; wherein the I/O device includes a PF (Physical Function), which can be allocated to one of the plurality of physical computers, and a plurality of VFs (Virtual Functions), which can be allocated to the plurality of LPARs; wherein the plurality of virtual machine monitors include a first virtual machine monitor to which the PF in the I/O device is allocated, and a second virtual machine monitor to which the PF is not allocated; wherein the first and second virtual machine monitors include VF execution means for executing a VF event for the plurality of VFs in the I/O device, which is allocated to the plurality of LPARs; and wherein the first virtual machine monitor includes PF execution means for executing a PF event for the PF in the I/O device, which is detected by the first or second virtual machine monitor.