1. An image forming apparatus for forming and recording an image on a recording medium according to an exposure value corresponding to an inputted diagnostic image signal, the image forming apparatus comprising: a recording medium loading section capable of loading plural types of recording media, each type of recording media having a different density range from the other; an exposing device for forming latent images on the recording medium by exposing the recording medium with predetermined plural test exposure values; a developing device for visualizing the latent images to form developed images on the recording medium; a density measuring device for measuring densities of the developed images corresponding to the predetermined plural test exposure values; a memory device for storing predetermined density values correlated to the predetermined plural test exposure values; a selecting device for selecting a predetermined number of images among the developed images corresponding to the predetermined plural test exposure values; and a calibration section for creating LUT that determines a relationship between input image signals and exposure values, based on measured densities of the selected predetermined number of images and stored predetermined density values correlated to the predetermined plural test exposure values.