1. A method of adjusting a density detecting device in an image forming apparatus, the image forming apparatus including a photoreceptor on which an electrostatic latent image is formed, a developing device for developing the electrostatic latent image formed on the photoreceptor into a toner image, and the density detecting device for irradiating light of a predetermined amount onto the photoreceptor to output density data corresponding to the amount of reflected light from the photoreceptor, the method comprising the steps of: successively irradiating light of amounts in a plurality of steps from a maximum amount to a minimum amount from the density detecting device onto one point of the photoreceptor on which no toner adheres, to acquire a plurality of density data outputted by the density detecting device which correspond to the amounts of light in the respective steps; selecting one density data on a predetermined basis out of the plurality of acquired density data corresponding to the amounts of light of the plurality of steps; irradiating light in the amount in the step corresponding to the selected one density data from the density detecting device onto a plurality of points of the photoreceptor, to acquire density data outputted by the density detecting device at the respective points; finding an average value of the acquired density data corresponding to the plurality of points as average density data; correcting the plurality of acquired density data corresponding to the amounts of light in the plurality of steps on the basis of the average density data and the selected one density data; and setting an amount of light to be irradiated onto the photoreceptor from the density detecting device when a density of a toner image formed on the photoreceptor is detected, on the basis of the plurality of the density data corrected.