1. An image forming apparatus, comprising: a photosensitive member; charging means which charges a surface of said photosensitive member; exposure means which irradiates a light beam over the surface of said photosensitive member which is charged by said charging means and accordingly forms an electrostatic latent image; developer means which visualizes said electrostatic latent image with toner and accordingly forms a toner image; and control means which optimizes a plurality of density adjustment factors each influencing the image densities of toner images and accordingly controls the image density of said toner image formed by said developer means, wherein said control means sets one piece of correlation data belonging to the product set of lower-density side correlation data and higher-density side correlation data, as optimal values of said plurality of density adjustment factors, said lower-density side correlation data being between said plurality of density adjustment factors and for forming a toner image at a lower-density side target density, said higher-density side correlation data being between said plurality of density adjustment factors and for forming a toner image at a higher-density side target density which is higher than said lower-density side target density.