Abstract: A grazing incidence spectrometer is provided with an entrance slit and a diffraction grating for diffracting the light rays incident thereon from the entrance slit and for imaging the light rays as a plurality of spectral lines. The diffraction grating is a curved diffraction grating having a predetermined principal radius of curvature and the groove patterns thereof are formed at unequal intervals so as to make the image plane thereof substantially planar. The entrance slit is disposed within a Rowland circle so as to satisfy0.7.ltoreq.r/R cos .alpha..ltoreq.0.9,where R is the principal radius of curvature of the curved diffraction grating, r is the distance between the entrance slit and the curved diffraction grating, and .alpha. is the angle of incidence of the principal ray incident from the entrance slit onto the curved diffraction grating.