PURPOSE: To minimize an access time for simultaneous input/output to/from two logical relative files by specifying converted relative key Nos. to corresponding physical relative files to input/output records.

CONSTITUTION: A record of a relative key No.1 is inputted/outputted to/from the 2nd logical relative file 12 by using a value '2' obtained from a pointer 153 indicating a file I/O control table in a control table 15 of the file 12 by a key conversion means as its relative key No.1. On the other hand, a record of a relative key No.1 is inputted/outputted to/from the 3rd logical relative file 13 by using a value 3 obtained from a pointer 163 indicating a file I/O control table in a control table 16 of the file 13 by the key conversion means as its relative key.