A storage system 103 carries out first and second de-duplication processes in response to receiving a write request from a client. First, a determination is made as to whether a write target data item overlaps with any of the stored data items of a part of a stored data item group, which is a user data item group stored in a storage device 209, and if so, the write target data item is prevented from being stored in the storage device. Second, a determination is made as to whether a target stored data item, which is not finished being evaluated as to whether it overlaps with the stored data item in the first de-duplication process, overlaps with another stored data item, and if so, the target stored data item or the same data item overlapping with the target stored data item is deleted from the storage device 209.