Snapshots are implemented by combining original data in a place where an operational volume has been updated with data in a place where the operational volume has not been updated. A snapshot management table maintains a value indicating that update has not been conducted, or a storage place of original data, for each of combinations of blocks in the operational volume and the snapshots. If there is a snapshot in which update has not been conducted in a update place at the time of update, then original data is copied and the snapshot management table is updated. The copied original data is managed by a difference block management table, which has a value indicating whether respective snapshots are referencing the data. If a snapshot is deleted, then the value indicating that the snapshot is referencing is altered in all entries in the difference block management table.