To make it possible to take over an IO configuration that is assigned to logical partitions in reallocation of the logical partitions, and to make an IO access work normally. A computer system has a server having an IO bridge, a switch that has a first IO bridge for connecting with the IO bridge of the server through a bus and plural second IO bridges for connecting to plural IO devices through a bus, and bus number assignment management means for fixedly assigning mutually different PCI bus numbers to the plural second IO bridges.