A processing unit is connected to another processing unit through a system bus composed of serial signal communication line and synchronization signal communication line to be able to communicate therewith. When an operation unit detects abnormal state in the processing unit, the operation unit supplies notification of detection of the abnormal state to synchronization unit. The synchronization unit transmits the received detection notification of abnormal state to the other processing unit through the synchronization signal communication line. Conversion unit receives parallel communication data from the operation unit through important signal line instead of general signal line and converts the received parallel signal into serial signal to be transmitted to the other processing unit through the serial signal communication line, thereby soundness among processing units connected to the system bus is ensured when the system bus is configured to attain serial communication.