Slices obtained by dividing the real data storage area of the storage device by segments are assigned to each of a plurality of segments obtained by dividing a virtual logical volume as a primary slice storing data of the segment as a destination of access made by an access node and/or a secondary slice that mirrors and stores data of the primary slice. Management information associates the segment with the primary slice and the secondary slice. A survival signal transmitted at predetermined intervals while a computer is normally operating is monitored. The computer from which the survival signal is not detected over a predetermined time period is detected as a failure node. The failure node is checked against the management information, the managed slice is set as a single primary slice that is an access destination of the access node for which the mirroring is stopped. The failure node is isolated.