A storage controller changes a block size to carry out a shredding process. A data shredder uses a large block size set by a block size setting part to write shredding data in a storage area of a disk drive and erase data stored therein. An error arising during the writing operation of the shredding data is detected by an error detecting part. When the error is detected, the block size setting part sets the block size smaller by one stage than the initial block size to the data shredder. Every time the error arises, the block size used in the shredding process is diminished. Thus, the number of times of writings of the shredding data is reduced as much as possible to improve a processing speed and erase the data of a wide range as much as possible.