PURPOSE: To set data in one output device by incorporating a priority determining circuit in output devices, allowing output devices to decide on their priority, and performing operation from a central processing unit as if only one output device were connected.

CONSTITUTION: The central processing unit 1 once knowing from an interruption level signal 7 that there is a request to output data generated by an output device 13 outputs a channel variation stop signal 16 to an output controller 14 through an output line 8 firstly to reset channel control FFs 19 of all output devices 13 and thus hold states of data request FFs 21 of all the output devices 13. Then, the priority of the output device 13 after said channel variation stop signal 16 is outputted is fixed and the data fetch permit gate 23 of only one output device 13 whose data input permit signal 22 is effective among output devices 13 having data request FFs 21 set is made effective. Then, the central processing unit 1 outputs data through the output line 8 and the output controller 14 outputs the data to a data bus 9.