An externally-connected volume of a main storage is correlated to an AOU volume inside of an external storage. The AOU volume is allocated with a not-yet-used page in a pool in accordance with data writing. When a command is issued to the externally-connected volume for formatting or others, a first controller in the main storage converts the command into a format command or an area deallocation command with respect to the AOU volume in the external storage. As such, the external AOU volume is subjected to a write process in its entirety, thereby being able to prevent any unnecessary page allocation. With such a configuration, the storage system of the present invention can use pages in the pool with good efficiency.