A processor determines a combination of the result of a comparison between a statistical value indicating the graininess of small dots and a predetermined reference value SDa and the result of a comparison between a statistical value indicating the graininess of large dots and a predicted value SDp. This combination shows which factors degrade image quality and which image forming conditions should be corrected and to what extent. Thus, the processor selects the image forming conditions to be corrected based on a combination of these comparison results, determines their correction values, and corrects the selected image forming conditions using the correction values. This makes it possible to properly correct the image forming conditions that correspond to the factors affecting image quality.