A light amount-adjusting device has a base plate with an optical path aperture, first and second blade members, and a driving unit for swinging open and close the blade members. Each blade member has a base-end portion and a front-end portion. The base-end portion is pivotally held on the base plate. The front-end portion faces the aperture. First and second boss-like protrusions project from the base plate and from the surfaces of the blade members. The protrusions are coupled to define the heightwise positions of the blade members in the direction of the optical axis. The vertically overlapping blade members pivotally supported on the base plate can be swung open and closed without interfering with each other. The device produces less frictional load when the aperture is opened and closed, and thus less noise.