Performance of a storage subsystem is monitored at a proper level in a computer system having host computers, the storage subsystems, and a management computer. Each storage subsystem has physical disks and a disk controller to control input/output of the data to/from the physical disks. The disk controller provides storage areas of the physical disks as one or more logical volumes to the host computer. The management computer makes each of the logical volumes correspond to one of a plurality of groups, sets monitoring conditions of the logical volumes every group, monitors performance information of the logical volumes in accordance with the set monitoring conditions, and if a correspondence between the logical volume and the group is changed, changes the monitoring conditions set to at least one of the plurality of groups.