PURPOSE: To attain high-speed inference without seeding a rule at extraction of a fault cause candidate by arranging in advance relating rules in setting the relating rules between individual fault and fault cause.

CONSTITUTION: A knowledge base pre-processing function 11 arranges the relation between a fault and its cause in a prescribed form at change in the rule. The upper half of the prescribed form shows the result listing up possible causes in each fault. Further, the lower half of the prescribed form shows the result listing up causes not requiring any consideration when no fault takes place. A hypothesis setting function 12a of a diagnosis executing function 12 uses data represented by the pre-processing in a prescribed procedure. That is, all causes # written on lines in the upper half of the pre-processing data corresponding to the observation inputted from a CRT are extracted. Then the observation is seeked from the lower half and all causes # written on corresponding lines, the extracted cause # is deleted to seek the final cause candidate.