1. A computer system capable of connecting a plurality of physical computers to an I/O (input/output) device through an I/O switch and sharing the I/O device; wherein the plurality of physical computers each include a plurality of logical partitions built on the physical computers, and a plurality of virtual machine monitors for managing the plurality of logical partitions; 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 logical partitions; 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 a VF execution section that executes a VF event for the plurality of VFs in the I/O device, which is allocated to the plurality of logical partitions; wherein the first virtual machine monitor includes a PF execution section that executes a PF event for the PF in the I/O device, which is detected by the first or second virtual machine monitor, wherein each of the plurality of logical partitions includes a VF driver for controlling the VF allocated to the logical partition; and wherein, when the first virtual machine monitor detects a VF event that is generated for a certain VF from the PF allocated to the first virtual machine monitor, the first virtual machine monitor notifies the second virtual machine monitor, which manages the logical partition to which the certain VF is allocated, of the VF event, allows the second virtual machine monitor to receive the notification of the VF event and transmit the VF event to the logical partition to which the certain VF is allocated, and causes the logical partition to execute the VF event by controlling the certain VF through the use of the VF driver possessed by the logical partition.