PURPOSE: To easily estimate the cause of a nonconformity evnt from a phenomenon generated incidentally to a generated failure event by using a relation table in which the relation of the phenomena representing the states of the failure events and the causes of the failure events is shown a numerical value.

CONSTITUTION: When a failure event is generated, steps to be taken are to collect various data 11 for representing the failure events and to execute a fault diagnosis based thereon by retrieving a data base 10 and checking the failure events caused in the past. The fault diagnosis consists of specifying 12 of the generated failure event, data fetching 13 for collecting a phenomenon generated incidentally to this failure event as numerical data and a sensory display, a diagnosis 14 for estimating the cause of generation of the failure event from this data and a relation table, the corresponding processing 15 to the diagnosed cause, and an output 16 of a diagnostic process for outputting a summary of the diagnostic process. The diagnostic process up to the fetching 13 of data are executed manually, and thereafter, they are executed by a computer.