An optical diaphragm for adjusting a light quantity of an incident light beam includes: a plurality of light-shielding vanes attached to a circumferential portion of an aperture of a base plate in a manner rotatable along a plane orthogonal to an optical axis of the light beam. The light-shielding vanes may be rotated to change an aperture area of the aperture of the base plate for transmitting the light beam. The plurality of light-shielding vanes each include a vane plate for intercepting the incident light beam; and a bearing integrally provided to the vane plate, the bearing each allowing a rotary shaft to be inserted therein so as to be supported by the rotary shaft. The vane plate is perpendicularly attached to the bearing. The plurality of light-shielding vanes move parallel to each other in an opening and closing direction thereof without crossing with each other.