A diaphragm device has a first base plate and a second base plate that are installed with a predetermined interval therebetween and formed into a unit. The diaphragm device is installed to a mounting portion of a lens frame such that the second base plate is disposed adjacently to a light source. The two base plates are partitioned with an intermediate plate to form two accommodating chambers, a stopping-down blade and a covering blade being disposed in each accommodating chamber. A motor is attached to the first base plate to relatively rotate a set of plate members and another set of plate members so as to restrict optical path apertures by the stopping-down blades. Portions directly exposed to light source light are provided with gray or white heat-resistant painting.