PURPOSE: To reduce a required storage area by checking whether or not the value of a classification item key has already been read in at every reading of a record from a file, and preserving the value of the classification item key in an internal storage only when it has not been read in.

CONSTITUTION: Firstly, a read-in key item value table 31 and a record number list 32 are generated by reading in an input file shown in figure. Next, the read-in key item value table 31, a leading pointer table 321, a tail pointer table 322 are classified simultaneously in ascending order setting the read-in key item value table 31 as a key. Finally, an equivalent result to the input of the file as shown in figure can be obtained logically by reading out a record number in the record number list 32 sequentially, and reading a corresponding record. In such a way, it is not always necessary to prepare an internal storage area being represented by the product of classification key item length and the number of records.