A failure diagnosis method diagnoses failure occurring in an image forming
apparatus. The failure causes defect in an image output from the image
forming apparatus. The failure diagnosis method includes acquiring
operation state signals indicating operation states during the image
forming apparatus operating in different operation conditions,
respectively; and analyzing the acquired operation state signals based on
a failure probability model, which is obtained by modeling a cause of the
failure occurring in the image forming apparatus with using
probabilities, to execute failure diagnosis with respect to each of
constituent members constituting the image forming apparatus.