PURPOSE: To realize continuous use by providing plural port parts, permitting a port control part to select the port parts and changing the connection of an external input/output processor to the other substitution port part when the port part breaks down.

CONSTITUTION: When an output processing is executed with the external input/ output processor 30 in a central processing unit 10, data from a main storage device 11 and an arithmetic processing unit 12 are transmitted to the port control part 21 through an input/output processor 13 and data are outputted by using a previously decided port part 22i (i≤n). For executing an input processing between the unit 10 and the processor 30, data from the processor 30 pass through all the port parts 221-22n and data from the port part 22i which the control part 21 selects is transferred to the device 11 and the unit 12 through the processor 13. When the fault of the port part 22i occurs, the control part 21 selects the substitution port part 22m (m≤n,m=1) based on the priority of scheduled selection and an input/output processing is continued.