PURPOSE: To attain ease of troubleshooting, by diagnosing the system in parallel with the instruction execution and storing the result.

CONSTITUTION: The inversion of ON/OFF of optional signals N0, N1WNn considered necessary for the diagnosis of failure from a logical circuit 1 is detected at a detecting circuit 4. The detected inverted signals DN0, DN1WDNn count inversion detecting counters 5, 6. The counted CN0, CN1WCNn and C2N0, C2N1W C2Nn are set to a signal recording register 7. On the other hand, the number of inverted times is set from a signal comparison information memory 11 recorded in advance to a comparison information register 9. Further, a comparison circuit 8 compares the information in the register 7 and that in the register 9. The compared information is discriminated at a discriminating circuit 13 and when they are dissident, the information of the counter 6 is recorded in a memory 15. Thus, the signal information considered to be a failure is recorded.