PURPOSE: To make it possible make the input/output device execute a stable operation at all times, by detecting the release of the electric power supply by the release detection device, and informing the computer of release current through the interruption generation device by the release current information device.

CONSTITUTION: In case when the electric power supply 8 has been released, the release current is detected by the release detection device 21, the release current is informed to the interruption generation device 7 by the release current information device 22, and a release current signal is converted to a computer signal. And its signal is sent to the computer 1 from the interface controller 2 in order to inform of the release current. At this time, the computer 1 receives the information of release current, and rewrites a data of the graphic pattern which has been broken down due to service interruption, to the CRT display unit 11. In this way, it is possible to make the input/output device execute a stable operation at all times.