PURPOSE: To attain high speed access to a permanent file by warranting a storage content of an external volatile storage device so as to build up the permanent file.

CONSTITUTION: The electronic computer system is provided with a file reserve means 2 reserving a permanent file to a nonvolatile external storage device simultaneously when the permanent file is reserved to a volatile external storage device, an input output control means 1 deciding input output destinations according to the volatile external storage device and implementing input output control and detecting an error in the volatile external storage device and recording it, and a file management table 3 storing the file information. Thus, read input output to the permanent file is implemented at a high speed by building up the permanent file to the volatile external storage device.