The present invention proposes a highly reliable storage system and data storage method that can prevent a data copying failure from occurring. If the allocation of a dynamically variable storage area to a primary/secondary logical volume from within that provided by a primary/secondary pool volume results in a storage area shortage in a primary pool volume, the storage system allocates an additional storage area to that primary pool volume from within the storage area provided by a physical volume, and together with this allocation, also allocates an additional storage area to all secondary pool volumes related to that primary pool volume.