In an ink-jet printing device which has an ink-jet recording head in which a plurality of nozzles are formed, and forms an image based on multi-level image data, a gamma correction parameter is selected according to discharging characteristics of the recording head to an input gradation level or according to lightness characteristics of a printed image to an input gradation level, and an image is formed based on the selected gamma correction parameter. For example, even when there is a variation in lightness characteristics of recording heads as in FIG. 9, an optimal gamma correction parameter is selected for each recording head and the image is formed. Thus, a constant image quality is obtained even if the recording heads are exchanged.