A mirror file number corresponding to a file being requested is transmitted to a host OS. A determination is made as to whether or not caching is performed in the host OS, and reading of the data of the mirror file number is requested to a block device if it is determined that the data of the mirror file number that was transmitted is not cached. The block device acquires the memory address where the corresponding actual contents are stored, corresponding to the sector number, acquires the sequence number corresponding to the contents, changes the acquired sequence number, and reads the data of the acquired memory address. The read data is converted to data with a modified sequence number attached, and the data is provided to the host OS. If it is determined that the data is cached in the host OS, the cached data is provided.