To inhibit deterioration in the I/O performance of a file even if the file includes an area that is frequently accessed.An access account of the file is recorded, an area of the file is split into a plurality of partial areas at least one or more times if the access count exceeds a split threshold, the partial area (partial area including a hotspot with frequent access) in which the access count exceeded the split threshold is minimized, data corresponding to that partial area is ultimately uncompressed, and data corresponding to the other partial areas is compressed.