A light amount adjusting apparatus includes: a two-dimensional (2D) aperture assembly that adjusts an opening area of a via hole through which light is transmitted; a three-dimensional (3D) aperture plate that forms a first opening and a second opening by blocking a part of the via hole, or moving to outside of the via hole to open the via hole; a first shielding plate that is movable to open or close the first opening; and a second shielding plate that is movable so as to close the second opening when the first shielding plate opens the first opening and to open the second opening when the first shielding plate closes the first opening.