PURPOSE: To improve the efficiency of repair work by returning contents of a correspondence table, where pairs of positions on a file system storage device and those on a storage device are stored, to the initial state in the case of destruction of data on a file system due to a failure of a program.

CONSTITUTION: When write to a file system 50 occurs, a first report means 2 reports write data and the write position to an output means 3, and the output means 3 stores data in an idle position on a storage device 30 and records a pair of the position on the storage device 39 and a position on a file system storage device 20 where data should be actually written. The pair recorded by the output means 3 is stored in a storage means 40. When read from the file system 50 occurs, a second report means 4 reports read contents to an input means 5 and discriminates whether the read position coincides with the position on the file system storage device 20 recorded in the storage means 40 or not, and data is read in from the storage device 30 in the case of coincidence, but data is read in from the file system storage device 20 in the case of disaccord. Thus, the man-hour of repair is reduced.