In a concave grating spectrometer for creating a spectral image by dispersing a light to be measured according to the wavelength, an arrangement comprising a concave grating with varied spacing and/or curved grooves formed on a concave spherical surface, and an entrance slit located at such a position on a vertical focal line passing through the center of curvature of the spherical surface, that the sum of the square of the difference between horizontal and vertical focal lengths at each wavelength in the wavelength range takes a minimum value. The light to be measured projected through the entrance slit is dispersed by the concave grating according to the wavelength and an image is focused on the vertical focal line passing through the center of curvature of the spherical grating.