To provide a software cache controlling method making it possible to improve the efficiency of write back and to efficiently use an area for a cache by preventing unnecessary writing operation in write back control.

When a request for reducing the size of the same file is given from a high-order software 5, file system information is changed and a block recording the pertinent data is registered in a cache controlling software 3 as writeback data. A block which is equivalent to a part reduced based on information managed by a file system managing software 4 and becomes an unused area is obtained and a block being the pertinent data is excluded from data held by the software 3. As a result, a data quantity held by the software 3 can be reduced.