A method and an apparatus for selecting an amount of light to be irradiated for fog detection onto a photoreceptor (10) from a density detecting device (23) out of amounts of light for low density or for high density. A real original (1) is illuminated by a predetermined amount of illuminating light, whereby a toner image corresponding to the real original (1) is formed on the photoreceptor (10). Light of an amount for low density is irradiated onto the toner image to acquire a first density data. In addition, a pseudo original (22a) is illuminated by the predetermined amount of illuminating light, whereby a toner image corresponding to the pseudo original (22a) is formed. Light of the amount for low density is irradiated onto the toner image to acquire a second density data. The difference between the first and the second density data is compared with a predetermined threshold value. If the difference is less than the threshold value, the amount of light for low density is selected for fog detection. If the difference is not less than the threshold value, the amount of light for high density is selected.