1. A failure diagnosis method for diagnosing failure occurring in an image forming apparatus, the failure causing defect in an image output from the image forming apparatus, the method comprising: acquiring operation state signals indicating operation states during the image forming apparatus operating indifferent 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.