A first optical scanning device for scanning image data of colors (yellow, magenta, cyan and black) which serve as the basic for formation of color image employs an optical system having optical characteristic stable to changes in temperature in common, the optical system being incorporated in a single optical box. Inexpensive resin material is employed for its lens in a second optical scanning device for scanning image data of pale color or a special color having a large tolerance amount to color deviation or spot diameter. Further, an inexpensive second optical scanning device saving space is formed by reducing the optical path length.