PURPOSE: To improve a sort executing processing efficiency by reading plural strings from an auxiliary memory device, using the same number of the input buffer as the number of the incorporate processing string and one preliminary buffer and executing the block forward looking of the string.

CONSTITUTION: An incorporate processing means 8 delivers the control to a forward looking control processing means 9, divides a memory part 3 into the same size of the block of a string, and sets plural input buffers 4W6 and one preliminary buffer 7. Next, at the beginning of respective incorporate processings, the forward looking control processing means 9 reads the head block of respective strings 10W12 from an auxiliary memory device 101 to input buffers 4W6 by using an input processing means 2, and thereafter, the final record in the block read to the input buffers 4W6 is removed, a sort key is compared, and sequencing is executed in accordance with the sort key. Thereafter, the forward looking control processing means 9 reads the next block from the string, which the block to complete fastest the incorporate processing belongs to, toward the preliminary buffer 7 by using the input processing means 2 and returns the control to the bulk processing means 8.