The present invention provides a measuring device by which, even if a radiation intensity from a light source, a beam size or a beam intensity distribution of the light source changes, an optical characteristic of an optical element to be measured can be measured very precisely. In a measuring device according to the present invention, to this end, light from a light source is diffracted by a diffracting grating to thereby resolve the same into plural light beams, and by using different light beams, the object to be measured is measured and the intensity of incident light from the light source is measured. With this structure, even if the light from the light source changes, the intensity of the light from the light source is specified concurrently, and therefore, the optical characteristic of the object to be measured can be measured very accurately.