1. An image forming apparatus comprising: an image forming part forming an image on a medium using developers of a plurality of colors; a sample pattern outputting part holding a sample pattern and supplying the held sample pattern to the image forming part to form an image on a medium to be output as a sample, a reference image and a plurality of color sample images being arranged in the sample pattern, the reference image being expressed by the plurality of the colors of predetermined densities, the plurality of the color sample images being formed by modifying a density of each of the plurality of the colors that are used to express the reference image by a predetermined amount; a selection part allowing a user to select one of the color sample images that configure the sample pattern; an image formation density adjustment amount holding part determining and holding image formation density adjustment amounts for increasing or decreasing image formation densities of the plurality of the colors that is used when the image forming part performs image formation, according to density distribution of the plurality of the colors that are used to express the color sample image that is selected by the user; an image formation density adjusting part adjusting image formation densities of the plurality of the colors of the image forming part, according to the image formation density adjustment amounts held by the image formation density adjustment amount holding part; a presenting part presenting to the user the image formation density adjustment amounts that are held by the image formation density adjustment amount holding part; and an image formation density adjustment amount modifying part receiving from the user modifications with respect to the image formation density adjustment amounts that are held by the image formation density adjustment amount holding part and updating the image formation density adjustment amounts that are held by the image formation density adjustment amount holding part to the image formation density adjustment amounts that is after the modification.