PURPOSE: To prevent the unnecessary input/output operation of temporary file data from an asynchronous input/output buffer to an external storage device from being generated when it is requested to close a temporary file.

CONSTITUTION: When a user program 10 issues the close request of a resident file as an input/output request 1a, an asynchronous input/output management table control means 12 writes resident file data, which are stored in an asynchronous input/output buffer 3 on a main storage device 4, through a control program 11 to an external storage device 5 by referring to an asynchronous input/ output buffer managing block 13. On the other hand, when the user program 10 issues the close request of the temporary file as the input/output request 1a, the asynchronous input/output management table control means 12 does not write temporary file data, which are stored in the asynchronous input/output buffer 3 on the main storage device 4, through the control program 11 to the external storage device 5 by referring to the asynchronous input/output managing block 13.