In a data conversion auxiliary module which is at a higher level than a file system in a disk management hierarchy, data stored in a storage medium, which becomes an object, is successively accessed. Then, a data conversion module captures a sector-unit access request to a device driver from the file system, converts data of a sector which is returned from the device driver, and writes the conversion data in the sector. Thereby, data conversion can be executed on a specific region of the storage medium, which is associated with the data in the storage medium.