An optical scanning device includes a rotationally-moving mirror on which a reflection plane is provided in a rotationally-movable manner, and a sealing unit over which a light-transmissive cover is provided, a first plane of the light-transmissive cover, a second plane of the light-transmissive cover, and the reflection plane of the rotationally-moving mirror within a rotationally-moving range being non-parallel to each other, the first plane being on the opposite side to the side of the rotationally-moving mirror, the second plane being on the side of the rotationally-moving mirror.