As an I/O method of issuing an I/O request, there are a synchronous I/O method, in which a task is in a waiting state until an I/O response to the I/O request is returned and an asynchronous I/O method, in which another task can be executed before an I/O response to the I/O request is returned. When issuing an I/O request in execution of a task, a DBMS selects any one of the synchronous I/O method and the asynchronous I/O method, based on an I/O response speed of an I/O destination storage device and issues an I/O request according to the selected I/O method.