PURPOSE: To perform operations, unit by unit, according to priority and to change the priority easily by providing an input/output control system with priority levels classified into two, i.e. one for interruption acceptance from an input/output device group and the other for control over the input/output device group.

CONSTITUTION: An input/output controller 3 is connected to a main storage device 1, processor 2, and input/output devices 10, and a microprocessor operates at plural priority levels. A level #0 is a top priority level for machine checking processing, a level #1 is for handling external interruption acceptance, and levels #2W#4 are for input/output control. Respective processes of the input/output devices 10 are stored in a control storage 8 by the levels and a processor performs operations of input/output devices with high levels in the storage 8. For example, the input/output devices 10-1 and 10-2 perform the same operation and their priority levels of the operation processing can be made different.