Abstract: An illuminating system for generating a high-intensity illuminating light utilizing a lamp of short arc type is equipped with a cooling device capable of effectively cooling the arc lamp and the reflecting optical members with an extremely simple structure. The illuminating system comprises a lamp emitting illuminating light of a high intensity; a reflecting optical member provided with a reflecting face surrounding the lamp and further provided at an end with a window for transmitting the light from the reflecting face and at the other end with an aperture for passing a part of the lamp; a casing integrally housing the reflecting optical member and the lamp and provided with a ventilating hole in a part of the casing for communication with the exterior; and air guide means for connecting the ventilating hole with an air path connecting the light transmitting window of the reflecting optical member with the aperture thereof.