An adjusting unit adjusts an output level of digital data converted from image data, based on a correction coefficient for each color. A color correction unit performs a color-correction process of correcting a fluctuation of adjusted output level based on a color-correction parameter that differs from one image reading device to another. The correction coefficient is generated before a reading unit is installed in the image reading device such that the output level becomes a predetermined value when reading the original document without the color-correction process, and then adjusted without the color-correction process at a time of installing the reading unit.