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.