Service life of a semiconductor flush memory is extended by averaging the number of times of rewrite operation and non-uniformity of time associated with a synchronization processing is eliminated. The invention provides a mapping rule such that when a space bitmap recorded to a semiconductor flush memory spreads over three erase blocks in the semiconductor flush memory using a UDF file system as a file system, a Root directory is recorded to partial DSA1 corresponding to partial SBM1, a first file directory group is recorded to only partial DSA1 and partial DSA2 corresponding to partial SBM1 and partial SBM2, and a second file directory group is recorded to only partial DSA1 and partial DSA3 corresponding to partial SBM1 and partial SBM3.