An I/O device management table that manages the types of I/O devices connected to an I/O switch is provided, and one or plural unallocated I/O devices are defined and registered as standby I/O devices. When a failure occurs in any of I/O devices, the I/O device management table is used to select an I/O device of the same type as the failed I/O device from the standby I/O devices, and the selected I/O device is allocated to a computer to which the failed I/O device is connected. I/O device management can be eased at failure in a computer including an I/O switch device.