PURPOSE: To quickly restore a fault by easily specifying the fault of a main storage device using the cache controller of a copy back system.

CONSTITUTION: At the time of detecting the occurrence of the fault of a main storage device 3 by a fault detecting means 11, a resident collection program 12 is activated. Then, the content of a cache memory 22 is compared with the content of the main storage device 3 corresponding to it by a comparing means 23. The compared result is stored in a specific area 31 of the main storage device 3 by a storing means 24. Then, the content of the main storage device 3 is swept out to a disk device 4, and the data stored in the specific area 31 of the main storage device 3 are read and analyzed, so that the analysis of the fault can be attained.