According to the present invention, a first lens part (60Y) and a second lens part (60C) in a first scanning lens (60) are vertically stacked so that lens centers thereof are to be separated from eachother by a first distance. A third lens part (61M) and a fourth lens part (61Bk) in a second scanning lens (61) are vertically stacked so that lens centers thereof are be separated from each other bya second distance equal to the first distance. The vertical positional relationship between the first lens part (60Y) and the second lens part (60C) is identical to the vertical positional relationship between the third lens part (61M) and the fourth lens part (61Bk). Further, the separating direction of the lens center of the first lens part (60Y) with respect to that of the second lens part (60C) is opposite to the separating direction of the lens center of the third lens part (61M) with respect to that of the fourth lens part (61Bk).