An I/O circuit system incorporated in the main controller of a semiconductor manufacturing apparatus or the like includes a self-diagnostic circuit in which tie switches are interposed between output channels which output control signals in order to drive and control apparatus-side driving portions constructed on an I/O board, and input channels which input return signals and sensor signals in response to the control signals, and self-diagnostic switches which disconnect power supply lines to the apparatus-side driving portions are arranged. To perform self-diagnosis upon generation of a fault or the like, the self-diagnostic switches are in a nonconductive state, and the tie switches are in a conductive state to electrically disconnect the apparatus. If a return signal corresponding to a self-diagnostic signal output from the main controller is returned, no electrical fault is determined to occur. If no return signal is returned, an electrical fault is determined to have occurred.