PURPOSE: To discriminate a faulty input/output controller, by displaying the faulty controller by a lamp and with the output of a flip-flop circuit which is provided at a fault discriminating circuit.

CONSTITUTION: A mode signal of a central controller CC, a data bus line 21 and a mode signal 22 of an input/output device IO are connected to the decoders 500, 501 and 502, respectively. FFs 515W518 are set based on the address of an input/output controller IOC given from the CC, the mode signal given from the CC and a transfer request signal given from each IOC. These FFs are then reset according to the end of the signal or the end of the received signal in the starting sequence of the CC. Then lamps 519W521 are lit up by the output of each FF. With such formation, a lmap corresponding to a faulty IOC is lit up. Thus the faulty IOC can be discriminated.