PURPOSE: To analyze in which data abnormality occurs by deciding that the abnormality occurs in at least a data block on one side out of the data blocks before and behind a chain when the chain to the data block is cut.

CONSTITUTION: The data blocks pointed by the data block B1 are P0, P1, and P2, and the data block pointed from a designated index to be the data block P2 is found out. The data blocks pointed to be the data block P2 are the data blocks D0, D1, and D2, and since the chain from the data blocks D1, D2 to another data block is cut, the data blocks D1, D2 are decided as abnormal data blocks, and it is checked whether or not an abnormal record is included by an operator by directly reading them out to a terminal side. At this time, when the data blocks D1 and D2 include the abnormal record, the data in the data blocks are corrected by an editor, etc.