An optical scanning device comprises an optical box that houses an optical component(s). The optical box includes a box main body and a cover member. A first engaging portion is formed on a side wall of the box main body, and a second engaging portion that engages with the first engaging portion is formed on a first fitting wall portion of the cover member. The box main body is provided with a vertical wall that is erected inside the first engaging portion. Moreover, there is formed on the cover member with a first barrier wall that is protruded downward from a top wall so as to enter a space between the first engaging portion and the vertical wall.