PURPOSE: To improve throughput with I/O simultaneous actions and to shorten an answer time by utilizing an actual value measured previously as the CPU application value totalized for a period during which the next I/O request is delivered from a program after a first I/O request is delivered and then deciding the dispatching priority.

CONSTITUTION: A dispatching control mechanism 20 is started again when the I/O execution is through to obtain the CPU application value totalized until the output of the next I/O request from an I/O control block table 31. While the corresponding entry of a task control block table 33 is searched from the task names 2 of the table 31. Thus the task priority is obtained. Furthermore the corresponding entry of an I/O time holding table 32 for average I/O time of each I/O device is searched from the device names 1 of the table 31. Thus the average I/O time of each I/O device is obtained. Then the dispatching priority is decided from the CPU application value, the task priority and the average I/O time respectively to rewrite a table 33.