PURPOSE: To enable random input/output processing from a user program to be performed by generating an intermediate file with attribute fitted in the file definition of the user program temporarily by comparing file attribute information on an auxiliary storage medium with the file definition of the user program.

CONSTITUTION: A file attribute recognizing means 21 obtains the file attribute information of an actually allocated file 31 before the file opening processing of a data processor 20 is started up for the open instruction 11 of the user program 10, and it is compared with the file definition of the program 10 by a file attribute comparison means 22. When it is judged that data conversion between them can be performed, the intermediate file 32 with attribute according to the file definition of the program 10 is generated temporarily by an intermediate file change means 23A, and the data conversion from the actually allocated file 31 to the intermediate file 32 is performed, and the file opening processing 25 on the intermediate file 32 is executed. Thereby, the random input/output processing by the user program can be performed.