A storage system comprises one or more pool volumes having chunks for storing data; one or more primary volumes; writable snapshots as virtual volumes for each primary volume which is a common ancestor of the writable snapshots, each primary volume and corresponding writable snapshots being members forming a snapshot group; and a storage controller which includes a processor, a memory storing, for each snapshot group, group information of the members within the snapshot group, and a deduplication module. The deduplication module may identify a snapshot group for deduplication based on the group information and perform deduplication of data for the identified snapshot group in a deduplication area, or perform deduplication of data in a deduplication area which is specified based on the group information of a snapshot group being generated in the storage system.