Provided is a storage system and a storage management method, aiming at reducing data movement amount necessary for using an expanded capacity in a distributed RAID. When only A (A is a positive integer) physical storage drives are added, a storage controller selects virtual parcels that are mapped to different physical storage drives among N physical storage drives and are included in different virtual chunks, changes an arrangement of the selected virtual parcels to the added A physical storage drives, and constitutes a new chunk based on unallocated virtual parcels selected from different physical storage drives among the (N+A) physical storage drives.