After passing through the process in which the number of one page I/O requests generated in a certain unit time period, the number of multiple pages I/O requests, and the number of pages required at each of the multiple pages I/O request are input, the quantity of the I/O requests generated in the above-described unit time period is calculated in accordance with the thus-input values. The unit (the number of pages) for a multiple pages I/O actually performed by one time operation of the I/O control device corresponding to the multiple pages I/O request from users is determined in accordance with the user's instruction or specific criterion for the system based on the result of the above-described calculation, the performance of the CPU, the performance of the I/O control device, the performance of the DASD (Direct Access Storage Decive), and to which DASD each of I/O requests corresponds. The result of this determination is employed as the unit (the number of pages) for the multiple pages I/O performed by one time operation of the I/O control device. In case of the specific criterion for the system, the structure may be constituted in such a manner that it can be automatically changed in accordance with the state of change in the quantity of the I/O request.