To shorten time required for sort processing at the time of deciding the sequence of data transmission and reception in asymmetrical subscriber's lines, and to simplify the predication of necessary memory quantity.

Combined data constituted of first and second elements are sorted by using the first and second elements as keys while the key constituting of the first elements is given priority. A chart having columns corresponding to all the possible combinations of the first elements and the second elements is prepared. Then, flags are erected in all the columns corresponding to inputted combined data (a, b). Then, the data (a) are scanned in the designated sequence, and one column of the data (a) are successively scanned in each scanning level in the sequence designated by the data b so that the columns (a, b) in which the flags are erected can be successively listed.