1. An image forming apparatus, comprising: an image carrier which has an endless shape and rotates in a predetermined direction, to thereby transport an electrostatic latent image which is carried on a surface of said image carrier; developing means which supplies toner to said electrostatic latent image, visualizes said electrostatic latent image with said toner and accordingly forms a toner image; and density detecting means which detects a toner density of a toner image which is formed as a patch image, wherein: while a density control factor, which influences an image density, set to be variable over multiple levels, a patch image is formed at each level of said image forming condition, said density detecting means detects toner densities of said patch images, and said density control factor is optimized based on the detection results; and a low-density patch image formed under a low-density side image forming condition, which makes an image density the lowest among said multiple levels of said image forming condition, has a length which is equal to or longer than a circumferential length of said image carrier in a patch length direction which corresponds to a direction in which said image carrier moves, said density detecting means detects a density in a portion of said low-density patch image which corresponds to said circumferential length of said image carrier, and a toner density of said low-density patch image is calculated.