An image forming apparatus comprising: - an image forming unit configured to form an image on paper based on a printing instruction, and to print the image; - an input luminance range recognition unit configured: i) to acquire input luminance information from multi-gradation monochrome image data in which a portion of an image area is selected, defining a selected image area, ii) to generate a luminance histogram from the input luminance information within the selected image area, and iii) to recognize, based on a minimum luminance value and a maximum luminance value of the luminance histogram, an input luminance range in which input luminance information of the selected image area is distributed; - an output luminance range recognition unit configured to recognize an output luminance range to be dealt with by the image forming unit; and - a luminance range expansion unit configured: i) to perform a mapping process for mapping input luminance information of pixels within the selected image area into the output luminance range, and ii) to expand the input luminance range to a maximum luminance range to be dealt with by the image forming unit, wherein the image forming apparatus further comprises: - a negative-positive inversion unit configured to perform negative-positive inversion process for input luminance information outside the selected image area; - a color conversion unit configured to convert after-expansion multi-gradation monochrome image data including the expanded input luminance information into CMYK serving as a color space of process colors of the image forming unit; and - a gamma correction processing unit configured to perform gamma correction on the CMYK-converted after-expansion multi-gradation monochrome image data. - an image forming unit configured to form an image on paper based on a printing instruction, and to print the image; - an input luminance range recognition unit configured: i) to acquire input luminance information from multi-gradation monochrome image data in which a portion of an image area is selected, defining a selected image area, ii) to generate a luminance histogram from the input luminance information within the selected image area, and iii) to recognize, based on a minimum luminance value and a maximum luminance value of the luminance histogram, an input luminance range in which input luminance information of the selected image area is distributed; i) to acquire input luminance information from multi-gradation monochrome image data in which a portion of an image area is selected, defining a selected image area, ii) to generate a luminance histogram from the input luminance information within the selected image area, and iii) to recognize, based on a minimum luminance value and a maximum luminance value of the luminance histogram, an input luminance range in which input luminance information of the selected image area is distributed; - an output luminance range recognition unit configured to recognize an output luminance range to be dealt with by the image forming unit; and - a luminance range expansion unit configured: i) to perform a mapping process for mapping input luminance information of pixels within the selected image area into the output luminance range, and ii) to expand the input luminance range to a maximum luminance range to be dealt with by the image forming unit, i) to perform a mapping process for mapping input luminance information of pixels within the selected image area into the output luminance range, and ii) to expand the input luminance range to a maximum luminance range to be dealt with by the image forming unit, - a negative-positive inversion unit configured to perform negative-positive inversion process for input luminance information outside the selected image area; - a color conversion unit configured to convert after-expansion multi-gradation monochrome image data including the expanded input luminance information into CMYK serving as a color space of process colors of the image forming unit; and - a gamma correction processing unit configured to perform gamma correction on the CMYK-converted after-expansion multi-gradation monochrome image data.