PURPOSE: To sharply reduce the necessary capacity of buffer parts expanded on a virtual storage by securing the buffer parts for themselves at the starting of the transaction processing of each application program.

CONSTITUTION: When an application program A outputs a read instruction, a control part 9A secures only a pointer 11A. At the starting of transaction processing, buffer parts 13-r, 13-s, 13-t, 13-u are secured on a main storage 4 on the basis of the contents of a working area controlling table 14 and respective addresses of these buffer parts are set in a pointer 11A to link the buffer parts with a control part 9A. After completing the processing corresponding to one transaction, changed contents out of the contents of the buffer part 13-4 or 13-u are stored in a data base storage part 15 or 18 on a data base 3 and the buffer part 13-r or 13-u is released and returned to the working area controlling table 14.