Provided is a computer system in which an I/O card is shared among physical servers and logical servers. Servers are set in advance such that one I/O card is used exclusively by one physical or logical server, or shared among a plurality of servers. An I/O hub allocates a virtual MM I/O address unique to each physical or logical server to a physical MM I/O address associated with each I/O card. The I/O hub keeps allocation information indicating the relation between the allocated virtual MM I/O address, the physical MM I/O address, and a server identifier unique to each physical or logical server. When a request to access an I/O card is sent from a physical or logical server, the allocation information is referred to and a server identifier is extracted from the access request. The extracted server identifier is used to identify the physical or logical server that has made the access request.