PURPOSE: To avoid the trouble of the interface change-over action by detecting that the interface connected to the change-over equipment is in the OFF line condition, executing actually the interface change-over action by the change-over equipment during the time and executing the diagnosing.

CONSTITUTION: The interface, which comes to be a change-over object, is provided with the means to detect the online/offline condition through a change-over equipment 6 to a change-over control mechanism 5a in a system supervisory equipment, the condition of a CPU corresponding to the time of offline is continued by an indicating means, the interface to the CPU corresponding to other interface is offline, the input output condition report is executed so that the diagnosis of the change-over equipment can be executed, when the interface comes to be offline, the change-over indication and the release indication are given as the diagnosis action of the change-over equipment 6 and it is confirmed that the correct change-over action is executed.