A first pattern is formed on a recording medium using a dark color substance and light color substance with substantially the same hue by supplying first pattern data to an image forming unit, and the density of the pattern is detected. If a variation in the detected density of the first pattern is not less than a predetermined value, conditions for image forming using the dark color substance and light color substance with the same hue in the image forming unit are corrected on the basis of the density of the first pattern and that of the first pattern data.