The present invention relates to a device for measuring light incident on an image-forming optical system, making use of a plural number of diffraction elements. The plural number of the diffraction elements are respectively provided in the optical path of the image-forming optical system. Further the plural number of the diffraction elements respectively have a certain predetermined area. The plural number of the diffraction elements, seen along the direction of the optical axis of the image-forming optical system, are substantially only partially overlapped with respect to each other. The light beam incident on a diffraction element is divided into a non-diffracted light beam and a diffracted light beam. Thus, the diffracted light beam is directed toward the light detecting means so as to be measured while the non-diffracted light beam advances along the above mentioned optical axis. Consequently, at the part at which the diffraction elements overlap each other, the non-diffracted light beam emerging from the preceeding diffraction element again enters the following diffraction element. This light beam is again divided into a non-diffracted light beam and a diffracted light beam by means of the following diffraction element. The diffracted light beam is measured by a light detecting means in the same way as in the above mentioned case, while the non-diffracted light beam advances along the above mentioned optical axis. Thus, the light beam which passes through the overlapping parts is measured a plural number of times.