PURPOSE: To restore a destroyed file speedily by providing group discrimination numbers to both pieces of end unit information on linkage among respective groups in a link structure file.

CONSTITUTION: A group discrimination number is the complementary number of a group number while zero is regarded as the base, and this is added to information 8 on the linkage from the starting unit information of final unit information and information 9 on the linkage from the final unit information of the starting unit information. If a fault occurs to an information controller 1 and a part of information is destroyed, the storage position of the starting unit information and final unit information in each group in unit information 6 are corrected into the starting storage position 3 and final storage position 4 of a group-classified starting and final unit information storage position 2. On the basis of the information 8, the order of the linkage is confirmed, group by group, and on the basis of the information 9, the order of the lnkage is also confirmed. Further, when improper information on the order of the linkage is found, the order is corrected into correct order.