When updating the data in each generation's logical units, the snapshot processing unit writes the update data in accordance with the update as forward differential data 221 to the base volume, and saves the old data that existed in the base volume before that update data was written as backward differential data in the backward differential data pool, and, when each generation's logical unit backward differential data is updated, deletes the forward differential data belonging the production LUs and the snapshots that together with those production LUs form the forward differential group. As a result of this, the amount of metadata necessary for executing a host command is able to be reduced in production LUs utilizing snapshots using the forward differential method.