A storage system 100, which has a plurality of flash packages 230, has a function for minimizing the imbalance of the number of deletions of each block inside the flash package 230 and a block-unit capacity virtualization function, and efficiently manifests lessening of the imbalance of the number of deletions and reduction in the data storage capacity for the entire storage system 100 by having functions for calculating the number of deletions and the data occupancy of each flash package 230, and for transferring data between the flash packages 230 on the basis of the values of these number of deletions and data occupancy.