There is a program for transmitting a prefetch command to a database management system (DBMS) so that access data of the DBMS can be prefetched to a storage. The DBMS is able to generate an access plan conscious of prefetch and to access database (DB). The DBMS is, however, unable to recognize that a prefetch command program is halted and it cannot generate an access plan at a proper I/O cost. Database management system servers 1, 2, . . . monitor a prefetch program of a management server 5, understands that the prefetch program is halted and it can optimize an execution plan to access storage apparatus 3, 4, . . . by using an I/O cost changed when the prefetch program is halted.