PURPOSE: To read out only file data which is needed by a file user in an information processing system having an auxiliary storage device.

CONSTITUTION: A requirement reception control part 11 receives the processing requirement of a file from the file user. When a requirement deciding control part 13 judges it to be a data writing requirement into the file, a level padding control part 16 pads a heading file data level and a data writing control part 14 writes the file. When the requirement deciding control part 13 judges the processing requirement to be a data reading requirement from the file, a data reading control part 15 reads out data from the file and a level discrimination control part 17 compares a required level and a level on file data and keeps reading-out data until file data satisfying the requirement is read out. Those are the feature of the device.