An optical scanning device is configured in such a manner that, of four light beams for colors corresponding to cyan, magenta, yellow, and black, the number of folding mirrors disposed in the optical path of a light beam corresponding to an mage in yellow having the highest brightness and the lowest visibility is the greatest number, and further, the number of the folding mirrors disposed in the optical path of a light beam corresponding to an image in black having the lowest brightness and the highest visibility is the least number. It is thus possible to reduce unevenness in color in the color image resulting from the folding of light beams by the folding mirrors.