An information system comprising: a system board having a first bridge device and a second bridge device, each being an I/O bus bridge device into which a logical bus number of an upstream-side bus that resides on its upstream side, and a logical bus number of a downstream-side bus that resides on its downstream side, are set; an active I/O board having multiple I/O bus bridge devices, each of the multiple I/O bus bridge devices of the active I/O board being set with a logical bus number of an upstream-side bus and a logical bus number of a downstream-side bus, wherein the active I/O board includes an I/O bus bridge device connected via a bus to the first bridge device of the system board; a standby I/O board, for being used in place of the active I/O board when the active I/O board fails, the standby I/O board having multiple I/O bus bridge devices, each of the multiple I/O bus bridge devices of the standby I/O board being set with a logical bus number of a upstream-side bus and a logical bus number of a downstream-side bus, wherein the standby I/O board includes an I/O bus bridge device connected via a bus to the second bridge device of the system board; and a configuration controller board being connected to the system board, the active I/O board and the standby I/O board via respective control buses, and having a capability to perform an I/O board replacement process that sets the logical bus number of the downstream side bus set in the first bridge device into the second bridge device after stopping operation of the first bridge device, and then sets the upstream-side and downstream-side logical bus numbers from each of the multiple I/O bus bridge devices of the active I/O board into corresponding multiple I/O bus bridge devices of the standby I/O board. a system board having a first bridge device and a second bridge device, each being an I/O bus bridge device into which a logical bus number of an upstream-side bus that resides on its upstream side, and a logical bus number of a downstream-side bus that resides on its downstream side, are set; an active I/O board having multiple I/O bus bridge devices, each of the multiple I/O bus bridge devices of the active I/O board being set with a logical bus number of an upstream-side bus and a logical bus number of a downstream-side bus, wherein the active I/O board includes an I/O bus bridge device connected via a bus to the first bridge device of the system board; a standby I/O board, for being used in place of the active I/O board when the active I/O board fails, the standby I/O board having multiple I/O bus bridge devices, each of the multiple I/O bus bridge devices of the standby I/O board being set with a logical bus number of a upstream-side bus and a logical bus number of a downstream-side bus, wherein the standby I/O board includes an I/O bus bridge device connected via a bus to the second bridge device of the system board; and a configuration controller board being connected to the system board, the active I/O board and the standby I/O board via respective control buses, and having a capability to perform an I/O board replacement process that sets the logical bus number of the downstream side bus set in the first bridge device into the second bridge device after stopping operation of the first bridge device, and then sets the upstream-side and downstream-side logical bus numbers from each of the multiple I/O bus bridge devices of the active I/O board into corresponding multiple I/O bus bridge devices of the standby I/O board.