PURPOSE: To reduce the explication error of the cause of the occurrence of failure by controlling each of plural input/output ports so as not to inform a diagnostic device of secondary failure in response to the report of the occurrence of the trouble of an input/output control processor.

CONSTITUTION: When the failure occurs at the internal part of the input/output control processor 4, it is informed to a diagnostic processor 3 through a diagnosis bus 102, and besides, a failure occurrence information signal is outputted to the input/output ports 5-i through a signal line 103. Since an output signal from an OR gate 55-1 is logical '0' and the output of a driver 56-1 is '0' as well if any other trouble does not occur in the input/output port 5-1, failure occurrence information from the input/output port 5-1 to the diagnostic processor 3 is not informed. Accordingly, in the indication/display processing of a device in which the failure occurs, the input/output port 5-1 is not indicated/displayed later than the input/output control processor 4. Thus, meaningless failure recovery processing is not performed in the diagnosis processor 3.